The Network Layer in the Internet• The Internet can be viewed as a collection of

subnetworks or Autonomous Systems (AS).• IP (Internet Protocol) hosts the whole Internet

together.• Communication in the Internet works as follows:

– The transport layer takes data streams and breaks them up into datagrams.

– Each datagram is transmitted through the Internet.

– When all the pieces finally get to the destination machine, they are reassembled by the network layer, which inserts it into the receiving process’ input stream.

IP Addresses

• IP addresses are the most common logical addresses. (Everyone on the Internet has one.)

• 32 - bit numbers (IP version 4)• 32 - bits yields 232 unique numbers• 232 = 4,294,967,2964,294,967,296

– there are over 4 billion possible IPv4 addresses– but many are “wasted” due to the allocation

scheme

IPv6: The Next GenerationThe newest version of IP (version 6, or IPng) uses 128 bits, yielding

2128 unique combinations

That’s over 340,000,000,000,000,000,000,000,000,000,000,000,000 possible addresses!

•IPv6 is slowly be integrated in the existing Internet.•IPv4’s 32 bits continues to be the dominant form of IP addressing.

The IP Header -V4

The IPv4 (Internet Protocol) header.

IPv4 Header Format• Version – The IP version number, 4. • Header length – The length of the datagram header in

32-bit words. • Type of service – Contains five subfields that specify the

precedence(priority 0-7), delay, throughput, reliability, and cost desired for a packet.

• Total length – The length of the datagram in bytes including the header, options, and the appended transport protocol segment or packet. The maximum length is 65535 bytes.

• Identification – An integer that identifies the datagram. • DF – Don’t fragment

IPv4 header format• MF – More Fragments. All fragments except the

last one have this bit set.• Fragment offset – The relative position of this

fragment measured from the beginning of the original datagram in units of 8 bytes.

• Time to live – How many routers a datagram can pass through. Each router decrements this value by 1 until it reaches 0 when the datagram is discarded. This keeps misrouted datagrams from remaining on the Internet forever.

• Protocol – The high-level protocol type.

IPv4 header format• Header checksum – A number that is computed to

ensure the integrity of the header values. • Source address – The 32-bit IPv4 address of the

sending host. • Destination address – The 32-bit IPv4 address of

the receiving host. • Options – A list of optional specifications for

security restrictions, route recording, and source routing. Not every datagram specifies an options field.

• Padding – Null bytes which are added to make the header length an integral multiple of 32 bytes as required by the header length field.

The IP Protocol

Some of the IP options.

5-54

IP Addresses• Traditionally, IP addresses were divided into

the five categories: A, B, C, D, E. • Network numbers are managed by a nonprofit

corporation called ICANN (Internet Corporation for Assigned Names and Numbers) to avoid conflicts.

• Network address, which are 32-bit numbers, are usually written in dotted decimal notation.

IP Addresses

IP address formats.

"Classful" Addressing Class Determination Algorithm

• If the first bit is a “0”, it's a class A address and we're done. (Half the address space has a “0” for the first bit, so this is why class A takes up half the address space.) If it's a “1”, continue to step two.

• If the second bit is a “0”, it's a class B address and we're done. (Half of the remaining non-class-A addresses, or one quarter of the total.) If it's a “1”, continue to step three.

• If the third bit is a “0”, it's a class C address and we're done. (Half again of what's left, or one eighth of the total.) If it's a “1”, continue to step four.

• If the fourth bit is a “0”, it's a class D address. (Half the remainder, or one sixteenth of the address space.) If it's a “1”, it's a class E address. (The other half, one sixteenth.)

12

Hosts for Classes of IP Addresses

Class A (24 bits for hosts) 224 - 2* = 16,777,214 maximum hosts

Class B (16 bits for hosts) 216 - 2* = 65,534 maximum hosts

Class C (8 bits for hosts) 28 - 2* = 254 maximum hosts

* Subtracting the network and broadcast reserved address

IP Addresses

Special IP addresses.

Subnets

A campus network consisting of LANs for various departments.

Subnetting • Division of a network into subnets

– For example, division of a Class B address into several Class C addresses

• Some of the host IDs are used for creating subnet IDs.

• Use parts of the host IDs for subnetting purpose• A subnet mask is used to facilitate the flow of

traffic between the different subnets and the outside network (hops).

• A hop is the distance a data packet travels form one node to the other

Routing of Traffic

140.15.0.0140.15.2.0

140.15.1.0

140.15.3.0

Routing

Subnets

1

2

3Outside world

Subnet Configuration

140 15 1 0

140 15 1 1

Subnet ID

First Host ID

140 15 1 254…..

Last Host ID

Subnetting Example• Consider the case of a class C address 195.

175.25.0 assigned to an organization• Subnets can be constructed by allocating part

of the higher-order bits of the host ID• Assume that three of the higher-order bits of

the host ID are to be reserved for that purpose

Subnetting Structure

195 175 25 0

11100000

Subnet Mask

Sub Net Last Octet Subnet ID1 00000000 195.175.25.

02 00100000 195.175.25.

323 01000000 195.175.25.

644 01100000 195.175.25.

965 10000000 195.175.25.

1286 10100000 195.175.25.

1607 11000000 195.175.25.

1928 11100000 195.175.25.

224

UsableSubnets(6)

Sample Subnet Division

Router

195.175.25.32 195.175.25.64

195.175.25.33...195.175.25.62

195.175.25.65...195.175.25.94

30 hosts per subnet.

Subnet 2Subnet 1

Overview of the Masking Process• IP address and subnet masks are used for the

masking operation• The purpose of masking is to identify whether

an IP address corresponds to a local host or a remote host

• The mathematical technique used is known as the ANDing process

Subnet Masking Example• Subnet ID: 195.175.25.32• Subnet Mask: 255.255.255.224 • Host address

– 195.175.25.34• Case 1 destination address

– 195.175.25.40• Case 2 destination address

– 195.175.25.67

Network Scenario

Router Subnet Mask: 255.255.255.224

Host195.175.25.34

Local Host195.175.25.40

OutsideWorld

195.175.25.40

195.175.25.67

Computing Subnet ID at Startup

Host ID 195 175 25 3411000011

10101111

00011001

00100010

Subnet Mask

255 255 255 22411111111

11111111

11111111

11100000

ANDingResult

195 175 25 3211000011

10101111

00011001

00100000Yields subnet ID.

TCP/IPPropertiesof the Host

Masking of Destination Address:Case 1

Destinati-nation IP

195 175 25 4011000011

10101111

00011001

00101000

Subnet Mask

255 255 255 22411111111

11111111

11111111

11100000

ANDingResult

195 175 25 3211000011

10101111

00011001

00100000

Yields subnet ID to be that of the local subnet.

Case 1 Forwarding of Data Packets• The destination host is local• Broadcast for the hardware address of the local

host at IP 195.175.25.40• Send information to the local host

Masking of Destination Address:Case 2

Destinati-nation IP

195 175 25 6711000011

10101111

00011001

01000011

Subnet Mask

255 255 255 22411111111

11111111

11111111

11100000

ANDingResult

195 175 25 6411000011

10101111

00011001

01000000

Yields subnet ID to be that of different subnet.

Case 2 Forwarding of Data Packets• The destination host is remote• Send information to the gateway • The router at the gateway will route the

data packet to the appropriate subnet

Summary of Transmission and Routing of Data Packets

Router Subnet Mask: 255.255.255.224

Host195.175.25.34

Local Host195.175.25.40

Subnet at 195.175.25.64

195.175.25.40(Case 1)

195.175.25.67(Case 2)

Subnet mask

A class B network subnetted into 64 subnets.

• For example, use a 6-bit subnet number and a 10-bit host number. The subnet mask is 255.255.252.0 or /22.

• Subnet addresses: 156.26.4.1, 156.26.8.1, 156.26.12.1, etc.

Supernetting

• Subnetting allows an organization to share a single IP network address among multiple physical networks

• Supernetting (a.k.a. classless addressing) allows the addresses assigned to an organization to span multiple IP network addresses

CIDR—Classless InterDomain Routing• For most organizations, a class A network, with

16 million addresses is too big, and a class C network, with 256 addresses is too small. A class B network, with 65,536, is just right.

• In Internet folklore, this situation is known as the three bears problem.

• The basic idea behind CIDR, is to allocate the remaining IP addresses in variable-sized blocks, without regard to the classes.

– If a site needs, say, 2000 addresses, it is given a block of 2048 addresses on a 2048-byte boundary.

– If need 8000 hosts, then allocate a block of 8192 addresses, i.e., 32 contiguous class C networks.

How a large number of IP addresses are wasted using IPv4 address classes?

• If a network has slightly more number of hosts than a particular class, then it needs either two IP addresses of that class or the next class of IP address.

• For example, let use say a network has 300 hosts, this network needs either a single class B IP address or two class C IP addresses.

• If class B address is allocated to this network, as the number of hosts that can be defined in a class B network is (2^16 - 2), a large number of host IP addresses are wasted.

• If two class C IP addresses are allocated, as the number of networks that can be defined using a class C address is only (2^21), the number of available class C networks will quickly exhaust.

• Because of the above two reasons, a lot of IP addresses are wasted and also the available IP address space is rapidly reduced

36

CIDR: classless interdomain routing Suppose an organization is allocated four contiguous class C networks: 205.100.0.0, 205.100.1.0, 205.100.2.0, 205.100.3.0.

Question: how to treat these four contiguous networks as one from outside?Network mask which will mask out one common prefix for these four networks.

CIDR is also called supernetting because it “supernets” multiple networks into one.

Question: what is the network mask for these four networks?

The common prefix: 11001101 . 01100100 . 000000

205.100.0.0 --- 11001101 . 01100100 . 00000000 . 00000000205.100.1.0 --- 11001101 . 01100100 . 00000001 . 00000000205.100.2.0 --- 11001101 . 01100100 . 00000010 . 00000000205.100.3.0 --- 11001101 . 01100100 . 00000011 . 00000000

Therefore, network mask: 11111111 . 11111111 . 111111 00 . 00000000, i.e., 255.255.252.0

In routing table, instead of putting all four networks entries, just put one entry:205.100.0.0/22, where 22 indicates the network mask is 22 bits.

CDR – Classless InterDomain Routing

A set of IP address assignments.

5-59

38

Private Network• Private IP network is an IP network that is not

directly connected to the Internet• IP addresses in a private network can be assigned

arbitrarily. – Not registered and not guaranteed to be globally

unique• Generally, private networks use addresses from

the following experimental address ranges (non-routable addresses): – 10.0.0.0 – 10.255.255.255– 172.16.0.0 – 172.31.255.255– 192.168.0.0 – 192.168.255.255

Note :Check http://10.45.10.4/

39

Private Addresses

H1

R1

H2

10.0.1.3

10.0.1.1

10.0.1.2

H3

R2

H4

10.0.1.310.0.1.2

Private network 1

Internet

H5

10.0.1.1Private network 1

213.168.112.3

128.195.4.119 128.143.71.21

40

Network Address Translation (NAT)• NAT is a router function where IP addresses

(and possibly port numbers) of IP datagrams are replaced at the boundary of a private network

• NAT is a method that enables hosts on private networks to communicate with hosts on the Internet

• NAT is run on routers that connect private networks to the public Internet, to replace the IP address-port pair of an IP packet with another IP address-port pair.

41

Basic operation of NAT

• NAT device has address translation table

H1

private address: 10.0.1.2public address: 128.143.71.21

H5

Privatenetwork

Internet

Source = 10.0.1.2Destination = 213.168.112.3

Source = 128.143.71.21Destination = 213.168.112.3

public address: 213.168.112.3NATdevice

Source = 213.168.112.3Destination = 128.143.71.21

Source = 213.168.112.3Destination = 10.0.1.2

PrivateAddress

PublicAddress

10.0.1.2 128.143.71.21

NAT – Network Address Translation

Placement and operation of a NAT box.

Internet Control Message Protocol• The control messages

– destination unreachable – time exceeded: TTL zero, (wandering to too long) – parameter problem: header invalid– source quench, too much packets (choke packet) – fragmentation required: MTU too small.

• for information messages: – echo request/reply– timestamp request/reply

• Two programs that use the ICMP protocol:– ping and traceroute

• IP invokes ICMP to report errors.

Internet Control Message Protocol

The principal ICMP message types.

5-61

How a host determines its IP address?

• A host determines its IP address during the boot-up process either from a configuration file stored in the local hard disk of the system

or• using a network protocol like RARP, DHCP,

BOOTP from the servers in the network.

ARP– The Address Resolution Protocol• ARP: Address Resolution Protocol

– find out the Ethernet address for an IP address – a host broadcast to everyone asking “who owns IP address

xxx.xxx.xxx.xxx”– The host with that IP address response with its Ethernet

address.• RARP: Reverse Address Resolution Protocol

– Find out a host’s IP address.– The host broadcast to everyone asking “My Ethernet

address is xx:xx:xx:xx:xx:xx, who knows my IP address?”– The RARP server looks up the configuration file and reply

with its IP address.

Dynamic Host Configuration Protocol• BOOTP (Bootstrap Protocol) is a protocol that lets a

network user be automatically configured (receive an IP address) and have an operating system booted (initiated) without user involvement. – Needs manually configuration (a table to map MAC to IP

address)

• DHCP (Dynamic Host Configuration Protocol) is a communications protocol that lets network administrators manage centrally and automate the assignment of IP addresses in an organization's network.

The Interior Gateway Routing Protocol

• The Internet is made up of a large number of autonomous systems(AS)

• Two-level routing:– interior gateway protocol – a routing algorithm

within an AS.– exterior gateway protocol – a routing algorithm

between Ases.

OSPF – Open Shortest Path First• OSPF supports three kinds of connections and

networks:1.Point-to-pint lines between exactly two routers.2.Multiaccess networks with broadcasting (e.g.,

most LANs.)3.Multiaccess networks without broadcasting (e.g.,

most packet-switched WANs).• OSPF represents the actual network as a graph like

this and then compute the shortest path from every router to every other router.

OSPF – The Interior Gateway Routing Protocol

(a) An autonomous system. (b) A graph representation of (a).

OSPF – The Interior Gateway Routing Protocol

• OSPF allows ASes to be divided into numbered areas, where an area is a network or a set of contiguous networks.

• Every AS has a backbone area (area 0). All areas are connected to the backbone.

• OSPF distinguishes four classes of routers:– Internal routers are wholly within one area.– Area border routers connect two or more areas.– Backbone routers are on the backbone– AS boundary routers talk to routers in other ASes.

OSPF

The relation between ASes, backbones, and areas in OSPF.

OSPF

The five types of OSPF messeges.

5-66

BGP – The Exterior Gateway Routing Protocol

• BGP (Border Gateway Protocol) is a protocol for exchanging routing information between gateway hosts (each with its own router) in a network of autonomous systems.

• BGP have been designed to allow many kinds of routing policies to be enforced in the interAS traffic.

BGP – The Exterior Gateway Routing Protocol

• Exterior gateway protocol routers have to worry about politics (security, billing, etc.)– BGP (Border Gateway Protocol) is essentially a

distance vector protocol.– But keep track of entire path.– Discard the route through itself solve count-to-

infinity.– Select route based on the distance (score). Any

route violating polices has infinite score and is discarded as it pass F.

BGP – The Exterior Gateway Routing Protocol

(a) A set of BGP routers. (b) Information sent to F.

Internet Multicating• IP supports multicasting, using class D addresses.• Two kinds of the group addresses are supported:

– Permanent groups: • 224.0.0.1: all system on a LAN• 224.0.0.2: all routers on a LAN• 224.0.0.5: all OSPF routers on a LAN• 224.0.0.6: all designated OSPF routers on a LAN

– Temporary groups must be created before used.• The query and response packets sent and received by

multicast routers are called IGMP (Internet Group Management Protocol). It has two kinds of packets: query and response.

• Multicasting routing is done using spanning tree.

IPv6

The Main IPv6 Header• Version. 4 bits. - IPv6 version number.• Traffic Class. 8 bits. - Internet traffic priority delivery

value.• Flow Label. 20 bits. - Used for specifying special router

handling from source to destination(s) for a sequence of packets.

• Payload Length. 16 bits, unsigned. - Specifies the length of the data in the packet. When set to zero, the option is a hop-by-hop Jumbo payload.

• Next Header. 8 bits. - Specifies the next encapsulated protocol. The values are compatible with those specified for the IPv4 protocol field.

The Main IPv6 Header• Hop Limit. 8 bits, unsigned. -For each router that

forwards the packet, the hop limit is decremented by 1. When the hop limit field reaches zero, the packet is discarded. This replaces the TTL field in the IPv4 header that was originally intended to be used as a time based hop limit.

• Source address. 16 bytes. - The IPv6 address of the sending node.

• Destination address. 16 bytes. -The IPv6 address of the destination node.

TCP/IP Protocol Suite 61

Figure 27.16 Format of an IPv6 datagram

TCP/IP Protocol Suite 62

Table 27.2 Table 27.2 Next header codesNext header codes

TCP/IP Protocol Suite 63

Table 27.3 Table 27.3 Priorities for congestion-controlled Priorities for congestion-controlled traffic traffic

TCP/IP Protocol Suite 64

Table 27.4 Table 27.4 Priorities for noncongestion-controlledPriorities for noncongestion-controlled traffic traffic

TCP/IP Protocol Suite 65

Table 27.5 Table 27.5 Comparison between IPv4 and IPv6 packet headerComparison between IPv4 and IPv6 packet header

IPv6• A new notation has been devised for writing 16-byte

addresses. • They are written as eight groups of four hexadecimal

digits with colons between the groups, like this:• 8000:0000:0000:0000:0123:4567:89AB:CDEF• Or 8000::123:4567:89AB:CDEF• one or more groups of 16 zero bits can be replaced by a pair

of colons

• IPv4 addresses can be written as a pair of colons 192.31.20.46

TCP/IP Protocol Suite 67

Figure 27.1 IPv6 address

TCP/IP Protocol Suite 68

Figure 27.2 Abbreviated address

TCP/IP Protocol Suite 69

Figure 27.3 Abbreviated address with consecutive zeros

IPv6 – Multicast and Anycast

IPv6 describes rules for three types of addressing: unicast (one host to one other host)anycast (one host to at least one of multiple

hosts), and multicast (one host to multiple hosts).

• The introduction of an "anycast" address provides the possibility of sending a message to the nearest of several possible gateway hosts with the idea that any one of them can manage the forwarding of the packet to others.