SYSTEM ADMINISTRATIONChapter 8

Internet Protocol (IP) Addressing

IPv4• IPv4 uses a 32-bit address space expressed in

binary format or decimal format.• Every IP address is composed of two parts: the

network number and the host number.• Every IP host address must be unique. Duplicate

addresses result in host devices that cannot communicate.

• Distribution of addresses is managed by IANA, which distributes addresses to regional agencies. The regional agency covering the United States is ARIN.

• ARIN allocates IP address blocks to the ISPs for distribution to customers.

IP Address Structure

• The 32 bits of an IPv4 address are broken into 4 8-bit groups called octets.

• Each octet is 1 byte of information.

• When an address is written in decimal format, the 4 octets are separated by a dot. This is known as dotted decimal notation.

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IP Address Structure(continued)

• In each octet, all 8 bits are assigned a binary value that is often expressed in decimal format. The decimal values, beginning with the most significant bit, are 128, 64, 32, 16, 8, 4, 2, and, 1 respectively.

• In binary format, bits are turned on or off to build the unique address for a node. Bits have a value of either 1 (on) or 0 (off).

• Conversion of binary to decimal or decimal to binary format can be done manually or through the use of the Windows calculator in Scientific mode.

IP Address Classes

• IPv4 addresses belong to one of five classes of addresses.• Class A addresses use the first octet (or 8 bits) to express

the network number, leaving the last three octets (or 24 bits) for host numbers. This allows 126 networks with 16,777,214 hosts on each network. The opening bit value is 0.

• Class B addresses use the first two octets (or 16 bits) for the network number, and the last two octets (or 16 bits) for the host number. This gives us 16,384 network numbers with 65,534 hosts on each network. The opening bit values are 10.

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IP Address Classes(continued)

• Class C addresses use the first three octets (or 24 bits) to indicate the network number, and the last octet (or 8 bits) for host numbers. There are 2,097,152 network numbers available, with 254 hosts on each of those networks. The opening bit value for Class C addresses is 110.

• Class D addresses are used for multicast transmission and are not manageable by the public. The opening bit value for a Class D address is 1110.

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IP Address Classes(continued)

• Class E addresses are used for experimental purposes. The opening bit value for a Class E address is 1111.

• Two addresses from every network range of addresses are reserved for special usage. One is the network number, and the other is used for broadcast purposes.

• Classful addressing uses the default values in each class for all IP addressing.

Private Addresses• Some addresses have been set aside to use on

private networks. Those address spaces are

o10.x.x.x-10.255.255.255

o172.16.x.x.-172.31.255.255

o192.168.x.x-192.168.255.255.

• Private addresses are not routed publicly. Therefore, if a router gets a packet with a private destination address in the header, the packet will not be routed and will be silently discarded.

Automatic Private IP Addressing (APIPA)

• Automatic Private IP Addressing (APIPA) provides a means for a DHCP client to self-configure when it cannot reach a DHCP server. APIPA uses the address space 169.254.0.X.

The Loopback Address• Another special address encompasses an entire

network number. 127.x.x.x is set aside for the loopback test. The loopback test sends a packet through the IP stack on the local host. If a response is received, the stack is installed and functioning.

• To assign addresses to host devices, the network administrator may configure every device manually, or the administrator may choose to use DHCP to allocate addresses to host devices.

IP Address Subnet Masks• A subnet mask identifies the bits that make up the

network number of an IP address. • The mask number is built by turning on (setting to 1) all

bits used for the network number. • The default subnet masks for the three consumer

classes of addresses are:o Class A – 255.0.0.0o Class B – 255.255.0.0o Class C – 255.255.255.0

• Another way to write the mask for an IP address is by using “slash notation.” Calculate the number of bits in the mask and follow the IP address with a slash and the number of bits: 202.16.22.45 /24.

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IP Address Subnet Masks(continued)

• The subnet mask also tells the local machine what to do with an outgoing packet. – The destination address is compared to the source

address, the two network numbers are compared, and the decision is made based on the result of the comparison.

– If the network numbers are the same, the packet is going to a host on the local subnet, and an ARP request is sent.

– If the network numbers are different, the packet is going to a remote network, and it will be forwarded to the default gateway (router interface) for processing.

• ANDING is the process by which the network number is resolved from an IP address by ANDING the mask against the host address

IP Address Subnetting• Subnetting is the process whereby a classful network

address may be broken down into several smaller subnetworks.

• Subnetted addresses add an additional level to the address space in the form of the subnet number.

• When designing a network that will include subnets, it is important to take into consideration the current needs of the network and host addresses, as well as the future needs of the network and host addresses.

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IP Address Subnetting(continued)

• There are five steps to subnetting:1. Determine the number of subnetworks needed and the

maximum number of hosts on any subnetwork.

2. Determine the number of bits to “steal” from the host bit portion of the IP address. Use the formula 2N – 2 = the maximum number of subnets available, where N = the number of bits to “steal.”

3. Build a new custom subnet mask to reflect the number of bits taken from the host section of the address and moved to the network section of the address.

4. Build the new network numbers.

5. Build the range of IP host addresses for each new subnet by combining the new network numbers with the identified range of host bits.

IP version 6 (IPv6)

• IPv6 in the newest version of the Internet Protocol. There are three key areas that make IPv6 the solution for the future:– A much larger address space using 128 bits as

opposed to 32 bits in IPv4– Built-in support for secure transmission through

the inclusion of IPSec in the protocol stack.– Support for mobile devices.

IPv6 Address Structure• IPv6 addresses are expressed as 8 16-bit fields written

in hexadecimal notation. • The address space is divided into a network number and

host number and may include a subnet identifier as well.• The host portion of the IPv6 address is always a fixed 64

bits in length.• The host portion is usually derived from the MAC

address of the device with filler bits in the center. This is known as the EUI64 address.

• Addresses may be abbreviated when there are a series of zeros in contiguous fields of the address. The abbreviator is a double colon (::), but this may occur only one time in the address. Leading zeros may be dropped.

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IPv6 Address Structure(continued)

• Three types of IPv6 addresses are used:– Globally unique addresses are the common

addresses of devices. They use the MAC address and the current network number to form the address. These addresses can be seen locally and globally (outside the subnet).

– Devices use the link-local address to identify themselves on the local subnet. These addresses are not visible outside the local subnet.

– Site-local addresses are used within a site and always read fec0:: /10. They are similar to the private addresses of IPv4.