An Introduction to IP Addresses

8/6/2019 An Introduction to IP Addresses

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An introduction to IP addresses: Part 1 by Kurt Keller

IP addresses (part 1) - on the internet you're only a number

If you have a connection to the internet and set it up yourself, you have at least oncehandled a number like 192.168.43.4. And every time you connect to the internet your computer becomes a number just like that one.

These numbers, consisting of four octets (or four bytes, a term which most people aremore familiar with) are called IP numbers. (And if you're curious what IP stands for, itmeans Internet Protocol.) IP numbers are used all over the internet, without them theinternet would not work. Even though they are all over the place, you rarely see them.One place you can find such a number for sure is the DNS server entry in your TCP/IP

setup. If you don't have a number in there, then you're either using DHCP (DynamicHost Configuration Protocol) to configure all these settings dynamically, you use IPnumbers exclusively, or you ought to enlighten me on how you can connect to theinternet without IP numbers.

DNS

An IP number, usually called IP address, is the address under which a specific machineon a TCP/IP network is known, similar to a phone number. Every machine has its ownaddress, sometimes even more than one. Also your computer gets assigned such anaddress when you connect to your provider. The provider will assign you an address out

of their pool temporarily and dynamically, unless you have a contract which guaranteesyou fixed IP addresses.

Humans, being somewhat different from computers, have quite some difficultyremembering lots of similar numbers. Working with names is much easier for us andless prone to errors. That's why you rarely see the bare numbers on the internet.Programs and computers are kind enough to translate names to numbers and back, sowe can conveniently work with what we can remember. This translation is done by theDomain Name System, in short: DNS. So the DNS server you specify in your TCP/IPsetup is the machine doing this translation for you. If you send off a request to connectto a certain website, let's say www.tokyopc.bbs.net, you first send a request to the DNS

server to lookup and translate www.tokyopc.bbs.net into the correct IP address for you.With this IP address you then make the actual connection to the website.

People were already using names instead of IP addresses before DNS came intoexistence in 1984. At that time, they had all the known machines entered in a text file,the so called 'hosts' file. This file has survived till today, even though it has not the sameimportance any more. Working with the hosts file only is possible only for a manageablenumber of connected machines. In the early days of the internet this was no problem.


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There was one master hosts file which everyone would retrieve regularly and copy totheir local machines. Whenever a host was connected newly to the internet or a hostwas taken off the net, a change in the master hosts file was necessary and all the other hosts only knew about the change after they retrieved the new master hosts file. Withthe internet growing rapidly, this way of managing name to IP resolution became

unmanageable; on one hand because of the sheer number of machines to manage andthe speed with which hosts were added, changed or removed, on the other handbecause of the enormous bandwidth consumed by everyone copying the ever bigger master hosts file to their local machines.

DNS, which was invented because of the aforementioned problems, is a sort of distributed database system and does some more things than simply translating namesinto addresses and vice versa, but going into details would be a topic of its own.

IP classes

If you happen to know the IP address of your provider's DNS server, the mailserver, thenews server and possibly some other machines, you will realize that very often the firstthree octets of their IP addresses are the same, for example 192.168.43.4 for the DNSserver, 192.168.43.5 for the mail server, 192.168.43.7 for the news server and192.168.43.25 for the secondary DNS server. This is not just by chance. Instead of giving out one IP address by one, there are classes which are assigned toorganizations. A, B, and C classes are the most known ones, with the C-class the mostcommon one. There are only 127 A-class ranges, or networks, but each of them has16,777,214 addresses for hosts. There are 16,384 possible B-class networks with65,534 addresses for hosts each and 2,097,152 C-class networks with 254 possiblehost addresses each.

Let's put it a little easier for the eye:

class networks hosts first octet

A 127 16'777'214 001 - 127

B 16'384 65'534 128 - 191

C 2'097'152 254 192 - 223

(The address range from 224.0.0.0 up is reserved for the special purpose classes D andE, which we will ignore here.)

Now you probably also understand why C-class addresses are the most common ones.First of all there are lots of them, and unless you are an institution with a couple of hundred thousand employees, you'll hardly be able to justify an A-class IP address.


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Looking at the numbers above, there must be some scheme to calculate the number of networks and hosts per class and there must be some difference for each class. Indeed,there is. Each IP address consists of four octets or bytes separated by dots, such as192.168.43.4. And in each address, there is a "class identifier," a "network part" and a"host part" encoded. For a C-class address, the host part is 1 byte long, for a B-class

address it is 2 bytes and for an A-class address it is 3 bytes. The class identifier for class A is 1 bit, for class B 2 bits and for class C 3 bits. A little more graphically, it lookslike this:

byte 1 byte 2 byte 3 byte 4

........ ........ ........ ........

0/////// -------- -------- -------- class A

10////// //////// -------- -------- class B

110///// //////// //////// -------- class C

1 0 actual values of the bits

/ bit used for network part

- bit used for host part

As you can see, a program can determine what class an IP address belongs to bychecking at which place the first bit with a value of 0 is. And according to this

information it then knows how many bits it should take for the network part of anaddress and how many bits for the host part. If we decode the address 192.168.43.4we'll find that it is a C-class IP address; 192.168.43 is the network part and 4 is the hostpart, so this is host 4 in the network 192.168.43.

192 168 43 4

11000000 10101000 00101011 00000100

class identifier

network part host part

I know you're already burning to know why a C-class only has 254 host addressesavailable, and not the full 256. If you calculated the number of hosts for the B- and A-classes too, you'll have realized that they are also smaller by two hosts from thetheoretical maximum. The answer is that the lowest address, the one with all bits in the


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host part set to 0 is used to address the network itself, and the highest address, havingall bits of the host part set to 1, is used as broadcast address, meaning all hosts in thisnet.

Everything clear? Ok then, we'll continue in the next issue with subnets.

Algorithmica Japonica Copyright Notice: Copyright of material rests with the individual author. Articlesmay be reprinted by other user groups if the author and original publication are credited. Any otherreproduction or use of material herein is prohibited without prior written permission from TPC. Themention of names of products without indication of Trademark or Registered Trademark status in no wayimplies that these products are not so protected by law.

An Introduction to IP Addresses: Part 2

by Kurt Keller

IP addresses (part 2) - one size does not fit all

Last time you got a brief explanation of what IP numbers are: the addresses for thehosts on a TCP/IP network like the internet. DNS (Domain Name System) is the hard-working guy looking up all the hostnames we throw at him and translating them to IPnumbers. And finally you saw how the classes of IP addresses are built. You probablyremember that the IP address range 192.0.0.0 - 223.255.255.255 is the C-class range,with 254 possible hosts per class and a total of 2,097,152 C-classes.

Subnets

What do you do if you own a C-class, but have five departments with 10 to 20 hostseach and need to have separate networks for each department? Do you need five C-classes? Luckily not. If that were necessary, we would long have run out of available C-classes (they are already very sparse now). You can manage your network with a singleC-class, using subnets. A subnet is a way of artificially making the network part of your IP address longer. And this can be done without telling everyone outside of your organization.

For subnetting, we need a so-called subnet mask. For a standard C-class IP address,the subnet mask is 255.255.255.0. The bits set to 1 (the first 24 bits in this case)represent the bits used for the network part and the bits set to 0 represent the part usedfor the host part:

192 168 43 4 IP address

11000000 10101000 00101011 00000100


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255 255 255 0 netmask

11111111 11111111 11111111 00000000

network part

host part

All the 1s in the netmask tell us that we take these bit positions of the IP address for thecalculation of the network part; all the 0's mask bits used for the host part. With this255.255.255.0 netmask, we have the standard 254 hosts and one single network in our C-class. If we want to implement subnets, we need to extend the network part and, atthe same time, decrease the host part. Extending the network part by 3 bits, we get asubnet mask of 255.255.255.224:

192 168 43 4 IP address

11000000 10101000 00101011 00000100

255 255 255 224 netmask

11111111 11111111 11111111 11100000

network part

host part

Now we have 27 bits for the network and only 5 bits for the host part, thus in our 192.168.43 C-class, we can have 8 subnets with 30 hosts each. This extension isknown to the machines on your own network only, any hosts outside of your organization do not know about it, nor do they need to know.

With the host part you already know that the all 0s and all 1s are special addresses andthus only two less than the theoretical maximum of hosts per subnet are usable. Witholder software, the same rule applies to subnets; the subnet number with all 0s in theadditional bits and the subnet number with all 1's can not be used. The network addressof the subnet with all 0s is the same as the standard network address of the wholeclass, and the broadcast address of the subnet with all 1s is the same as the standard

broadcast address of the whole class. This restriction is obsolete by now, but you maystill encounter software which can not use these two border subnets.

For clarity let's quickly list the infos of our created subnets:

with a standard C-class netmask of 255.255.255.0:

net hosts broadcast


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192.168.43.000 192.168.43.001 - 192.168.43.254 192.168.43.255

with a custom netmask of 255.255.255.224:

net hosts broadcast

!192.168.43.000! 192.168.43.001 - 192.168.43.030 192.168.43.031

192.168.43.032 192.168.43.033 - 192.168.43.062 192.168.43.063

192.168.43.064 192.168.43.065 - 192.168.43.094 192.168.43.095

192.168.43.096 192.168.43.097 - 192.168.43.126 192.168.43.127

192.168.43.128 192.168.43.129 - 192.168.43.158 192.168.43.159

192.168.43.160 192.168.43.161 - 192.168.43.190 192.168.43.191

192.168.43.192 192.168.43.193 - 192.168.43.222 192.168.43.223

192.168.43.224 192.168.43.225 - 192.168.43.254 !192.168.43.255!

Using subnetting, the number of addressable hosts decreases, because you havemultiple network and broadcast addresses, but for many purposes it is well worthimplementing subnets.

Just two more remarks:

Network bits for subnets do not need to be consecutive, you could well use255.255.255.130 (11111111.11111111.11111111.10000010) but it does no good,unless you want to make things much more complicated than they already are. Yougain nothing with such non-standard subnet masks; the number of usable nets and thenumber of usable hosts is exactly the same as with subnet masks using consecutivebits.

Here we only looked at subnets with fixed length subnet masks, that means the samesubnet mask is used over the whole network. It is, however, possible to use differentsubnet masks for different parts of your network (called "variable length subnet masks"),

which means that you can splice and dice your assigned network into subnets to exactlymeet your needs. However, older software and routing protocols may not understandvariable length subnet masks.

Well, this is enough heavy stuff for today. Give it some time to settle before going onwith something about routing and CIDR in the next issue.


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Algorithmica Japonica Copyright Notice: Copyright of material rests with the individual author. Articlesmay be reprinted by other user groups if the author and original publication are credited. Any otherreproduction or use of material herein is prohibited without prior written permission from TPC. Themention of names of products without indication of Trademark or Registered Trademark status in no wayimplies that these products are not so protected by law.

Introduction to TCP/IP

Summary: TCP and IP were developed by a Department of Defense (DOD) researchproject to connect a number different networks designed by different vendors into anetwork of networks (the "Internet"). It was initially successful because it delivered a fewbasic services that everyone needs (file transfer, electronic mail, remote logon) across avery large number of client and server systems. Several computers in a smalldepartment can use TCP/IP (along with other protocols) on a single LAN. The IPcomponent provides routing from the department to the enterprise network, then toregional networks, and finally to the global Internet. On the battlefield a communications

network will sustain damage, so the DOD designed TCP/IP to be robust andautomatically recover from any node or phone line failure. This design allows theconstruction of very large networks with less central management. However, because of the automatic recovery, network problems can go undiagnosed and uncorrected for longperiods of time.

As with all other communications protocol, TCP/IP is composed of layers:

y IP - is responsible for moving packet of data from node to node. IP forwards each packet basedon a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to

departments. IP operates on gateway machines that move data from department toorganization to region and then around the world.y TC P - is responsible for verifying the correct delivery of data from client to server. Data can be

lost in the intermediate network. TCP adds support to detect errors or lost data and to triggerretransmission until the data is correctly and completely received.

y Sockets - is a name given to the package of subroutines that provide access to TCP/IP on mostsystems.

Ne twork of Low es t Bidd e r s

The Army puts out a bid on a computer and DEC wins the bid. The Air Force puts out a

bid and IBM wins. The Navy bid is won by Unisys. Then the President decides to invadeGrenada and the armed forces discover that their computers cannot talk to each other.The DOD must build a "network" out of systems each of which, by law, was delivered bythe lowest bidder on a single contract.


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The Internet Protocol was developed to create a Network of Networks (the "Internet").Individual machines are first connected to a LAN (Ethernet or Token Ring). TCP/IPshares the LAN with other uses (a Novell file server, Windows for Workgroups peer systems). One device provides the TCP/IP connection between the LAN and the rest of the world.

To insure that all types of systems from all vendors can communicate, TCP/IP isabsolutely standardized on the LAN. However, larger networks based on long distancesand phone lines are more volatile. In the US, many large corporations would wish toreuse large internal networks based on IBM's SNA. In Europe, the national phonecompanies traditionally standardize on X.25. However, the sudden explosion of highspeed microprocessors, fiber optics, and digital phone systems has created a burst of new options: ISDN, frame relay, FDDI, Asynchronous Transfer Mode (ATM). Newtechnologies arise and become obsolete within a few years. With cable TV and phonecompanies competing to build the National Information Superhighway, no singlestandard can govern citywide, nationwide, or worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely within the currenttechnological uncertainty. TCP/IP data can be sent across a LAN, or it can be carriedwithin an internal corporate SNA network, or it can piggyback on the cable TV service.Furthermore, machines connected to any of these networks can communicate to anyother network through gateways supplied by the network vendor.

A ddr esses

Each technology has its own convention for transmitting messages between twomachines within the same network. On a LAN, messages are sent between machinesby supplying the six byte unique identifier (the "MAC" address). In an SNA network,every machine has Logical Units with their own network address. DECNET, Appletalk,and Novell IPX all have a scheme for assigning numbers to each local network and toeach workstation attached to the network.


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On top of these local or vendor specific network addresses, TCP/IP assigns a uniquenumber to every workstation in the world. This "IP number" is a four byte value that, byconvention, is expressed by converting each byte into a decimal number (0 to 255) andseparating the bytes with a period. For example, the PC Lube and Tune server is130.132.59.234.

An organization begins by sending electronic mail to [email protected] assignment of a network number. It is still possible for almost anyone to getassignment of a number for a small "Class C" network in which the first three bytesidentify the network and the last byte identifies the individual computer. The author followed this procedure and was assigned the numbers 192.35.91.* for a network of computers at his house. Larger organizations can get a "Class B" network where thefirst two bytes identify the network and the last two bytes identify each of up to 64thousand individual workstations. Yale's Class B network is 130.132, so all computerswith IP address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen regional or specialized network suppliers. The network vendor is given the subscriber networknumber and adds it to the routing configuration in its own machines and those of theother major network suppliers.

There is no mathematical formula that translates the numbers 192.35.91 or 130.132 into"Yale University" or "New Haven, CT." The machines that manage large regionalnetworks or the central Internet routers managed by the National Science Foundationcan only locate these networks by looking each network number up in a table. There arepotentially thousands of Class B networks, and millions of Class C networks, butcomputer memory costs are low, so the tables are reasonable. Customers that connect

to the Internet, even customers as large as IBM, do not need to maintain anyinformation on other networks. They send all external data to the regional carrier towhich they subscribe, and the regional carrier maintains the tables and does theappropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and the Red Sox. Inthis spirit, Yale recently switched its connection from the Middle Atlantic regionalnetwork to the New England carrier. When the switch occurred, tables in the other regional areas and in the national spine had to be updated, so that traffic for 130.132was routed through Boston instead of New Jersey. The large network carriers handlethe paperwork and can perform such a switch given sufficient notice. During aconversion period, the university was connected to both networks so that messagescould arrive through either path.

S ubn e t s

Although the individual subscribers do not need to tabulate network numbers or provideexplicit routing, it is convenient for most Class B networks to be internally managed as amuch smaller and simpler version of the larger network organizations. It is common to


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subdivide the two bytes available for internal assignment into a one byte departmentnumber and a one byte workstation ID.

The enterprise network is built using commercially available TCP/IP router boxes. Eachrouter has small tables with 255 entries to translate the one byte department number into selection of a destination Ethernet connected to one of the routers. Messages to thePC Lube and Tune server (130.132.59.234) are sent through the national and NewEngland regional networks based on the 130.132 part of the number. Arriving at Yale,the 59 department ID selects an Ethernet connector in the C& IS building. The 234selects a particular workstation on that LAN. The Yale network must be updated as newEthernets and departments are added, but it is not effected by changes outside the

university or the movement of machines within the department. A Unc e rtain Path

Every time a message arrives at an IP router, it makes an individual decision aboutwhere to send it next. There is concept of a session with a preselected path for alltraffic. Consider a company with facilities in New York, Los Angeles, Chicago and

Atlanta. It could build a network from four phone lines forming a loop (NY to Chicago toLA to Atlanta to NY). A message arriving at the NY router could go to LA via either Chicago or Atlanta. The reply could come back the other way.

How does the router make a decision between routes? There is no correct answer.Traffic could be routed by the "clockwise" algorithm (go NY to Atlanta, LA to Chicago).The routers could alternate, sending one message to Atlanta and the next to Chicago.More sophisticated routing measures traffic patterns and sends data through the leastbusy link.

If one phone line in this network breaks down, traffic can still reach its destinationthrough a roundabout path. After losing the NY to Chicago line, data can be sent NY to


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Atlanta to LA to Chicago. This provides continued service though with degradedperformance. This kind of recovery is the primary design feature of IP. The loss of theline is immediately detected by the routers in NY and Chicago, but somehow thisinformation must be sent to the other nodes. Otherwise, LA could continue to send NYmessages through Chicago, where they arrive at a "dead end." Each network adopts

some Router Protocol which periodically updates the routing tables throughout thenetwork with information about changes in route status.

If the size of the network grows, then the complexity of the routing updates will increaseas will the cost of transmitting them. Building a single network that covers the entire USwould be unreasonably complicated. Fortunately, the Internet is designed as a Networkof Networks. This means that loops and redundancy are built into each regional carrier.The regional network handles its own problems and reroutes messages internally. ItsRouter Protocol updates the tables in its own routers, but no routing updates need topropagate from a regional carrier to the NSF spine or to the other regions (unless, of course, a subscriber switches permanently from one region to another).

Undiagno se d Probl ems

IBM designs its SNA networks to be centrally managed. If any error occurs, it isreported to the network authorities. By design, any error is a problem that should becorrected or repaired. IP networks, however, were designed to be robust. In battlefieldconditions, the loss of a node or line is a normal circumstance. Casualties can be sortedout later on, but the network must stay up. So IP networks are robust. Theyautomatically (and silently) reconfigure themselves when something goes wrong. If there is enough redundancy built into the system, then communication is maintained.

In 1975 when SNA was designed, such redundancy would be prohibitively expensive, or it might have been argued that only the Defense Department could afford it. Today,however, simple routers cost no more than a PC. However, the TCP/IP design that,"Errors are normal and can be largely ignored," produces problems of its own.

Data traffic is frequently organized around "hubs," much like airline traffic. One couldimagine an IP router in Atlanta routing messages for smaller cities throughout theSoutheast. The problem is that data arrives without a reservation. Airline companiesexperience the problem around major events, like the Super Bowl. Just before thegame, everyone wants to fly into the city. After the game, everyone wants to fly out.Imbalance occurs on the network when something new gets advertised. Adam Curry

announced the server at "mtv.com" and his regional carrier was swamped with trafficthe next day. The problem is that messages come in from the entire world over highspeed lines, but they go out to mtv.com over what was then a slow speed phone line.

Occasionally a snow storm cancels flights and airports fill up with stranded passengers.Many go off to hotels in town. When data arrives at a congested router, there is noplace to send the overflow. Excess packets are simply discarded. It becomes theresponsibility of the sender to retry the data a few seconds later and to persist until it


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finally gets through. This recovery is provided by the TCP component of the Internetprotocol.

TCP was designed to recover from node or line failures where the network propagatesrouting table changes to all router nodes. Since the update takes some time, TCP is

slow to initiate recovery. The TCP algorithms are not tuned to optimally handle packetloss due to traffic congestion. Instead, the traditional Internet response to trafficproblems has been to increase the speed of lines and equipment in order to say aheadof growth in demand.

TCP treats the data as a stream of bytes. It logically assigns a sequence number toeach byte. The TCP packet has a header that says, in effect, "This packet starts withbyte 379642 and contains 200 bytes of data." The receiver can detect missing or incorrectly sequenced packets. TCP acknowledges data that has been received andretransmits data that has been lost. The TCP design means that error recovery is doneend-to-end between the Client and Server machine. There is no formal standard for

tracking problems in the middle of the network, though each network has adopted somead hoc tools.

Nee d to Know

There are three levels of TCP/IP knowledge. Those who administer a regional or national network must design a system of long distance phone lines, dedicated routingdevices, and very large configuration files. They must know the IP numbers andphysical locations of thousands of subscriber networks. They must also have a formalnetwork monitor strategy to detect problems and respond quickly.

Each large company or university that subscribes to the Internet must have anintermediate level of network organization and expertise. A half dozen routers might beconfigured to connect several dozen departmental LANs in several buildings. All trafficoutside the organization would typically be routed to a single connection to a regionalnetwork provider.

However, the end user can install TCP/IP on a personal computer without anyknowledge of either the corporate or regional network. Three pieces of information arerequired:

1 . The IP address assigned to this personal computer

2. The part of the IP address (the subnet mask) that distinguishes other machines on the same LAN(messages can be sent to them directly) from machines in other departments or elsewhere inthe world (which are sent to a router machine)

3. The IP address of the router machine that connects this LAN to the rest of the world.

In the case of the PCLT server, the IP address is 130.132.59.234. Since the first threebytes designate this department, a "subnet mask" is defined as 255.255.255.0 (255 isthe largest byte value and represents the number with all bits turned on). It is a Yale


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convention (which we recommend to everyone) that the router for each departmenthave station number 1 within the department network. Thus the PCLT router is130.132.59.1. Thus the PCLT server is configured with the values:

y M y IP address: 1 30. 1 32.59.234y

S

ubnet mask: 255.255.255.0y Default router: 1 30. 1 32.59. 1

The subnet mask tells the server that any other machine with an IP address beginning130.132.59.* is on the same department LAN, so messages are sent to it directly. AnyIP address beginning with a different value is accessed indirectly by sending themessage through the router at 130.132.59.1 (which is on the departmental LAN).

Additional information is available in self-study courses from SRA (1-800-SRA-1277)

Introduction

Previous Next

This talk will cover the basics of IP addressing and subnetting.Topics covered will include:

y W hat is an IP Address?y W hat are Classes?y W hat is a Network Address?y W hat are Subnet M asks and Subnet Addresses?y H ow are Subnet M asks defined and used?y H ow can all this be applied?y W hat is CIDR?y H ow can I get more information?

N ote : This tutorial is designed to be viewed on line. Ihave received numerous requests to have a text-only or single-page printable version of theTutorial. N o such document exists. If you need ahard copy of this Tutorial, you can print each pagein turn with your browser's print function. I hopethis is sufficient for most people.


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IP A ddr ess ingAn IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network.An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotteddecimal" notation.

Example: 140.179.220.200

It is sometimes useful to view the values in their binary form.

140 .179 .220 .20010001100.10110011.11011100.11001000

Every IP address consists of two parts, one identifying the network and one identifyingthe node. The Class of the address and the subnet mask determine which part belongsto the network address and which part belongs to the node address.

A ddr ess Cla sses There are 5 different address classes. You can determine which class any IP address is in by examiningthe first 4 bits of the IP address.

y Cla ss A addresses begin with 0xxx , or 1 to 126 decimal.y Cla ss B addresses begin with 10xx , or 128 to 191 decimal.y Cla ss C addresses begin with 110x , or 192 to 223 decimal.y Cla ss D addresses begin with 1110 , or 224 to 239 decimal.y Cla ss E addresses begin with 1111 , or 240 to 254 decimal.

Addresses beginning with 01111111 , or 127 decimal, are reserved for loopback and for internal testing on a local machine; [You can test this: you should always be able to ping127.0.0.1 , which points to yourself] Class D addresses are reserved for multicasting;Class E addresses are reserved for future use. They should not be used for hostaddresses.

Now we can see how the Class determines, by default, which part of the IP addressbelongs to the network ( N, in blue ) and which part belongs to the node ( n, in red ).

y Class A -- NNNNNNNN.nnnnnnnn .nnnnnnnn .nnnnnnnn y Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn .nnnnnnnn y Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn

In the example, 140.179.220.200 is a Class B address so by default the Network part of the address (also known as the Network Address ) is defined by the first two octets(140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).

In order to specify the network address for a given IP address, the node section is set toall "0"s. In our example, 140.179.0.0 specifies the network address for


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140.179.220.200. When the node section is set to all "1"s, it specifies a broadcast thatis sent to all hosts on the network. 140.179.255.255 specifies the example broadcastaddress. Note that this is true regardless of the length of the node section.

Privat e S ubn e t s

There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0, SubnetMa sk 255.0.0.0 , 172.16.0.0, Subnet M a sk 255.240.0.0 , and 192.168.0.0, Subnet M a sk 255.255.0.0.These addresses are also notated 10.0.0.0/8 , 172.16.0.0/12 , and 192.168.0.0/16 ; this notation will beexplained later in this tutorial. They can be used by anyone setting up internal IP networks, such as a labor home LAN behind a NAT or proxy server or a router. It is always safe to use these because routers onthe Internet by default will never forward packets coming from these addresses. These addresses aredefined in RFC1 91 8.

Subnetting

Subnetting an IP Network can be done for a variety of reasons, including organization, use of different

physical media (such as Ethernet, FDDI, W AN, etc.), preservation of address space, and security. Themost common reason is to control network traffic. In an Ethernet network, all nodes on a segment seeall the packets transmitted by all the other nodes on that segment. Performance can be adverselyaffected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used toconnect IP networks to minimize the amount of traffic each segment must receive.

S ubn e t Ma s king

Applying a subnet mask to an IP address allows you to identify the network and nodeparts of the address. The network bits are represented by the 1s in the mask, and the

node bits are represented by the 0s. Performing a bitwise logical AND operationbetween the IP address and the subnet mask results in the Network Address or Number.For example, using our test IP address and the default Class B subnet mask, we get:

10001100.10110011.11110000.11001000 140.179.240.200 Class B IP Address11111111 . 11111111 . 00000000 . 00000000 255.255.000.000 Default Class BSubnet Mask--------------------------------------------------------10001100.10110011.00000000.00000000 140.179.000.000 Network Address

Default subnet masks:

y Cla ss A - 255 .0.0.0 - 11111111 .00000000 .00000000 .00000000 y Cla ss B - 255 .255 .0.0 - 11111111 .11111111 .00000000 .00000000 y Cla ss C - 255 .255 .255 .0 - 11111111 .11111111 .11111111 .00000000

M ore Restrictive Subnet M asks

Additional bits can be added to the default subnet mask for a given Class to further subnet, or break down, a network. When a bitwise logical AND operation is performed between the subnet


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mask and IP address, the result defines the Subnet Address (also called the Network Address or Network Number ). There are some restrictions on the subnet address. Node addresses of all "0"sand all "1"s are reserved for specifying the local network (when a host does not know its network address) and all hosts on the network (broadcast address), respectively. This also applies tosubnets. A subnet address cannot be all "0"s or all "1"s. This also implies that a 1 bit subnet mask

is not allowed. This restriction is required because older standards enforced this restriction.Recent standards that allow use of these subnets have superseded these standards, but many"legacy" devices do not support the newer standards. If you are operating in a controlledenvironment, such as a lab, you can safely use these restricted subnets.

To calculate the number of subnets or nodes, use the formula (2 n-2) where n = number of bits ineither field, and 2 n represents 2 raised to the nth power. Multiplying the number of subnets by thenumber of nodes available per subnet gives you the total number of nodes available for your class and subnet mask. Also, note that although subnet masks with non-contiguous mask bits areallowed, they are not recommended.

Example:10001100.10110011.11011100.11001000 140.179.220.200 IP Address11111111 . 11111111 . 111 00000 . 00000000 255 . 255 . 224 . 000 Subnet Mask--------------------------------------------------------10001100.10110011.11000000.00000000 140.179.192.000 Subnet Address10001100.10110011.11011111.11111111 140.179.223.255 Broadcast Address

In this example a 3 bit subnet mask was used. There are 6 (2 3-2) subnets available with this sizemask (remember that subnets with all 0's and all 1's are not allowed). Each subnet has 8190 (2 13-2) nodes. Each subnet can have nodes assigned to any address between the Subnet address andthe Broadcast address. This gives a total of 49,140 nodes for the entire class B address subnetted

this way. Notice that this is less than the 65,534 nodes an unsubnetted class B address wouldhave.

You can calculate the Subnet Address by performing a bitwise logical AND operation betweenthe IP address and the subnet mask, then setting all the host bits to 0s. Similarly, you cancalculate the B roadcast Address for a subnet by performing the same logical AND between theIP address and the subnet mask, then setting all the host bits to 1 s. That is how these numbers arederived in the example above.

Subnetting always reduces the number of possible nodes for a given network. There are completesubnet tables available here for Class A , Class B and Class C . These tables list all the possible

subnet masks for each class, along with calculations of the number of networks, nodes and totalhosts for each subnet.

An Example

H ere is another, more detailed, example. Say you are assigned a Class C network number of 200.133.175.0 (apologies to anyone who may actually own this domain address). You want to


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utilize this network across multiple small groups within an organization. You can do this bysubnetting that network with a subnet address.

We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes onthe network instead of the 254 we would have without subnetting, but gives us the advantages of

traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long.Recall that the default Class C subnet mask is

255 . 255 . 255 . 0 ( 11111111 . 11111111 . 11111111 . 00000000 binary)Extending this by 4 bits yields a mask of 255.255.255. 240 ( 11111111 . 11111111 . 11111111 . 1111 0000 binary)

This gives us 16 possible network numbers, 2 of which cannot be used:

S ubnet bits Network Number Node Addresses Broadcast Address

0000 200.133.175.0 Reserved None

0001 200.133.175.16 .17 thru .30 200.133.175.31

0010 200.133.175.32 .33 thru .46 200.133.175.47

0011 200.133.175.48 .49 thru .62 200.133.175.63

0100 200.133.175.64 .65 thru .78 200.133.175.79

0101 200.133.175.80 .81 thru .94 200.133.175.95

0110 200.133.175.96 .97 thru .110 200.133.175.111

0111 200.133.175.112 .113 thru .126 200.133.175.127

1000 200.133.175.128 .129 thru .142 200.133.175.143

1001 200.133.175.144 .145 thru .158 200.133.175.159

1010 200.133.175.160 .161 thru .174 200.133.175.175

1011 200.133.175.176 .177 thru .190 200.133.175.191

1100 200.133.175.192 .193 thru .206 200.133.175.207

1101 200.133.175.208 .209 thru .222 200.133.175.223

1110 200.133.175.224 .225 thru .238 200.133.175.239

1111 200.133.175.240 Reserved None

CIDR -- Cla ss less Int e rDo m ain Routing Now that you understand "classful" IP Subnetting principals, you can forget them ;). The reasonis CIDR -- C lassless Inter Domain R outing. CIDR was invented several years ago to keep theinternet from running out of IP addresses. The "classful" system of allocating IP addresses can bevery wasteful; anyone who could reasonably show a need for more that 254 host addresses wasgiven a Class B address block of 65533 host addresses. Even more wasteful were companies and


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organizations that were allocated Class A address blocks, which contain over 16 Million hostaddresses! Only a tiny percentage of the allocated Class A and Class B address space has ever

been actually assigned to a host computer on the Internet.

People realized that addresses could be conserved if the class system was eliminated. By

accurately allocating only the amount of address space that was actually needed, the addressspace crisis could be avoided for many years. This was first proposed in 1992 as a scheme calledS upernetting . Under supernetting, the classful subnet masks are extended so that a network address and subnet mask could, for example, specify multiple Class C subnets with one address.For example, If I needed about 1000 addresses, I could supernet 4 Class C networks together:

192.60.128.0 (11000000.00111100.10000000.00000000) Class C subnet address192.60.129.0 (11000000.00111100.10000001.00000000) Class C subnet address192.60.130.0 (11000000.00111100.10000010.00000000) Class C subnet address192.60.131.0 (11000000.00111100.10000011.00000000) Class C subnet address--------------------------------------------------------192.60.128.0 (11000000.00111100.10000000.00000000) Supernetted Subnetaddress255.255.252.0 ( 11111111 . 11111111 . 111111 00 . 00000000 ) Subnet Mask192.60.131.255 (11000000.00111100.10000011.11111111) Broadcast addressIn this example, the subnet 192.60.128.0 includes all the addresses from 192.60.128.0 to192.60.131.255. As you can see in the binary representation of the subnet mask, the Network

portion of the address is 22 bits long, and the host portion is 10 bits long.

Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of spellingout the bits of the subnet mask, it is simply listed as the number of 1s bits that start the mask. Inthe above example, instead of writing the address and subnet mask as

192.60.128.0, Subnet Mask 255.255.252.0

the network address would be written simply as:192.60.128.0/22which indicates starting address of the network, and number of 1s bits (22) in the network

portion of the address. If you look at the subnet mask in binary(11111111 .11111111 .111111 00.00000000 ), you can easily see how this notation works.

The use of a CIDR notated address is the same as for a Classful address. Classful addresses caneasily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24)

It is currently almost impossible for an individual or company to be allocated their own IPaddress blocks. You will simply be told to get them from your ISP. The reason for this is the

ever-growing size of the internet routing table. Just 10 years ago, there were less than 5000network routes in the entire Internet. Today, there are over 100,000. Using CIDR, the biggestISPs are allocated large chunks of address space (usually with a subnet mask of /19 or evensmaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the bigISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessiblevia 1 network route on the Internet. But I digress.


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It is expected that CIDR will keep the Internet happily in IP addresses for the next few years atleast. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy addressallocation would comfortably allow a billion unique IP addresses for every person on earth! Thecomplete and gory details of CIDR are documented in RFC1519 , which was released inSeptember of 1993.

Allowed Class A Subnet and H ost IP addresses

# bits S ubnet Mask CIDR # S ubnets # Hosts Nets * Hosts

2 255.192.0.0 /10 2 4194302 8388604

3 255.224.0.0 /11 6 2097150 12582900

4 255.240.0.0 /12 14 1048574 14680036

5 255.248.0.0 /13 30 524286 15728580

6 255.252.0.0 /14 62 262142 162528047 255.254.0.0 /15 126 131070 16514820

8 255.255.0.0 /16 254 65534 16645636

9 255.255.128.0 /17 510 32766 16710660

10 255.255.192.0 /18 1022 16382 16742404

11 255.255.224.0 /19 2046 8190 16756740

12 255.255.240.0 /20 4094 4094 16760836

13 255.255.248.0 /21 8190 2046 16756740

14 255.255.252.0 /22 16382 1022 16742404

15 255.255.254.0 /23 32766 510 16710660

16 255.255.255.0 /24 65534 254 16645636

17 255.255.255.128 /25 131070 126 16514820

18 255.255.255.192 /26 262142 62 16252804

19 255.255.255.224 /27 524286 30 15728580

20 255.255.255.240 /28 1048574 14 14680036

21 255.255.255.248 /29 2097150 6 12582900

22 255.255.255.252 /30 4194302 2 8388604

Allowed Class B Subnet and H ost IP addresses


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2 255.255.192.0 /18 2 16382 32764

3 255.255.224.0 /19 6 8190 49140

4 255.255.240.0 /20 14 4094 57316

5 255.255.248.0 /21 30 2046 61380

6 255.255.252.0 /22 62 1022 63364

7 255.255.254.0 /23 126 510 64260

8 255.255.255.0 /24 254 254 64516

9 255.255.255.128 /25 510 126 64260

10 255.255.255.192 /26 1022 62 63364

11 255.255.255.224 /27 2046 30 61380

12 255.255.255.240 /28 4094 14 57316

13 255.255.255.248 /29 8190 6 49140

14 255.255.255.252 /30 16382 2 32764

Allowed Class C Subnet and H ost IP addresses


2 255.255.255.192 /26 2 62 124

3 255.255.255.224 /27 6 30 180

4 255.255.255.240 /28 14 14 196

5 255.255.255.248 /29 30 6 180

6 255.255.255.252 /30 62 2 124

Logical Operations

This page will provide a brief review and explanation of the common logical bitwise operations AND, OR,XOR (Exclusive OR) and NOT. Logical operations are performed between two data bits (except for NOT).Bits can be either " 1 " or "0", and these operations are essential to performing digital math operations.In the "truth tables" below, the input bits are in bo ld , and the results are plain.


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A N DThe logical AND operation compares 2 bits and if they are both " 1 ", then the result is " 1 ", otherwise, theresult is "0".

0 1

0 0 0

1 0 1

O RThe logical OR operation compares 2 bits and if either or both bits are " 1 ", then the result is " 1 ",otherwise, the result is "0".

0 1

0 0 1

1 1 1

XO RThe logical XOR (Exclusive OR) operation compares 2 bits and if exactly one of them is " 1 " (i.e., if theyare different values), then the result is " 1 "; otherwise (if the bits are the same), the result is "0".

0 1

0 0 1

1 1 0

N O TThe logical NOT operation simply changes the value of a single bit. If it is a " 1 ", the result is "0"; if it is a"0", the result is " 1 ". Note that this operation is different in that instead of comparing two bits, it isacting on a single bit.

0 1

1 0

Documents

An Introduction to IP Addresses