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IP Addresses

Based Computer Networks and Internets (Comer)

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IP Address

The logical (and thus software) destination is denoted by an IP Address.

The IP Address provides homogeneity over diverse networks.

The IP Address allows a host to change hardware (and thus hardware address) and yet still be found at the software level.

Also IP Addresses are assigned in a more logical manner, which can facilitate routing.

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Prefix/Suffix

The routing is facilitated by the fact that an IP Address is hierarchical.

It consists of two parts: The first part of the address (starting on the left)

is known as the prefix and it identifies the host’s network (the group of computers it belongs to).

The second part of the address is known as the suffix and it identifies the individual computer (node) within the above specified network.

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IP Addressing Scheme

In IP(v4) each host is assigned a unique 32-bit number which is the address for the host.

To transmit on a TCP/IP internet, a host must know its own IP address as well as that of the destination. We’ll qualify the above statement later.

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Dotted Decimal Notation

Each 8-bit section (known as a byte or octet) of the 32-bit number [IP(v4)] is expressed as a decimal value with periods between them.

The combination of eight bits can be in 256=28

states, which are expressed as the numbers 0 through 255.

The range of valid addresses which can be assigned is 0.0.0.0 to 255.255.255.255, which barring various reserved addresses is 4,294,967,296=232

The La Salle network is 139.84.0.0 , the computer www.lasalle.edu is 139.84.10.250

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Uniqueness

Each computer on an internet (or on the Internet) must have a unique address. Two hosts on the same internet can have

different network portions and the same node portion.

Two hosts on the same internet can have the same network portion and different node portions.

Two hosts on the same internet can have different network portions and different node portions.

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Address Classes

The IP address is not divided into equal halves with one half for the network portion, the other for the node potion

The addressing scheme tries to accommodate for the fact that Some networks (not many) will contain a vast

number of hosts While other networks (very many) will contain a

more modest number of hosts Thus the IP Class system was developed.

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The IP Class System

Originally the IP(v4) Addresses were broken into 5 classes: A through E.

The dividing line between network portion and node portion of the IP address differed from class to class.

The first four bits on the left identify the class to which an address belongs.

A, B and C are the primary classes for the addressing, D and E were reserved.

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Reserved Class Class D is used for multicasting. Class D

addresses begin with the first four bits 1110. If a set of hosts uses multicasting, they agree to

share the multicast address. When a message is transmitted to the multicast

address, each host in the group makes a copy. Class E is reserved for future use. Class E

addresses begin with the first four bits 1111.

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Figure from Comer’s book

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Class A

Class A is self-identified by the leftmost bit being a 0.

Class A uses the first octet from the left to identify the network and the rest to identity the nodes. It has 7 bits (first octet minus first bit used to

indicate class A) to identify networks, so there can be 128 = 27 Class A networks.

It has 24 bits (the last three octets) to identify nodes, so there can be 16777216=224 nodes on a Class A network (almost).

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Reserved Addresses

Actually the node addresses consisting of all 1’s and all 0’s are reserved, so the number of Class A nodes is actually

16,777,214=224 – 2 All 0’s (in the suffix) is reserved to refer

to the network itself. All 1’s (in the suffix) is used to

broadcast on the network.

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Class B

Class B is self-identified by the first two bits being a 10.

Class B uses the first two octets from the left to identify the network and the rest to identity the nodes It has 14 bits (first two octet minus first two bits

used to indicate class B) to identify networks, so there can be 16384 = 214 Class B networks.

It has 16 bits (the last two octets) to identify nodes, so there can be 65534=216 –2 nodes on a Class B network.

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Class C

Class C is self-identified by the first three bits being a 110.

Class C uses the first three octets from the left to identify the network and the remaining one to identity the nodes. It has 21 bits (first three octet minus first three bits

used to indicate class C) to identify networks, so there can be 2097152 = 221 Class C networks.

It has 8 bits (the last octet) to identify nodes, so there can be 254 =28 –2 nodes on a Class C network.

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These are all off by 2 because it is neglected by node addresses (suffixes) reserved for the network and broadcasting

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Computing Address Class

In Class A, the first octet starts with a 0, thus the smallest number is

And the largest number (in Class A) is

So in decimal-dot notation, Class A addresses start with a number between 0 and 127

0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1

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Computing Address Class

In Class B, the first octet starts with a 10, thus the smallest number is

And the largest number is

So in decimal-dot notation, Class B addresses start with a number between 128 and 191

www.lasalle.edu (139.84.10.250) is Class B

1 0 0 0 0 0 0 0

1 0 1 1 1 1 1 1

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Computing Address Class

In Class C, the first octet starts with a 110, thus the smallest number is

And the largest number is

So in decimal-dot notation, Class C addresses start with a number between 192 and 223

1 1 0 0 0 0 0 0

1 1 0 1 1 1 1 1

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subnet

A Class B network (like LaSalle’s) can have 65534 hosts.

To manage traffic within the network, it is useful to break the network into sub-networks (subnets).

On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix.

But Class B has a two-octet prefix, so wouldn’t all 65534 of the hosts be on the same subnet?

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Subnet mask

Dividing a network into subnets is useful for both security and performance reasons.

The division of the prefix and suffix portions was made more flexible by using subnet masks.

In addition to setting an IP address, one also sets a subnet mask which specifies which portion of the address is used to identify the network and which portion is used to identify the hosts within a network.

The 1’s in the subnet mask correspond to the network part, the 0’s correspond to the host part.

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Subnet mask

By convention, the bits for the network address are all set to 1 It would also work if the bits were set exactly

the same as in the network address (prefix). A typical subnet mask looks like

11111111.11111111.11110000.00000000.

One extracts the subnet address by performing a bitwise AND operation on the mask and the IP address.

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CIDR

The use of subnet masking instead of classes sometimes goes by the name Classless Inter-Domain Routing (CIDR).

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Special IP Addresses Network Addresses: IP reserves the host (suffix)

address of 0’s for a network Direct Broadcast: IP reserves the host (suffix) address

of 1’s for broadcasting within the network Limited Broadcast: Used during system startup by a

computer that does not know its IP address. The entire prefix and suffix are assigned all 1’s for the local network.

This Computer: Used by a computer to define its address. The computer needs to send or receive packets to determine its address on the network. This happens during startup. IP reserves the address of all zeros to mean the initial host computer.

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LoopBack

Used to test network applications. Can test computer-to-computer applications using one computer by forcing a packet down through the protocol stack by using the loopback address.

The application uses the loopback address to send the data to “another” application which is running on the same machine.

During loopback testing, no packets leave the computer.

The network prefix of 127 is reserved for loopback. Any suffix is used. (Reduces the number of Class A networks.)

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Ping localhost

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Berkeley Broadcast Address Form TCP/IP was distributed with a version of

Unix put out by Berkeley. In that version of TCP/IP, all 0’s in the

suffix is used for as broadcasting (instead of all 1’s).

Many people used this version, so now there are standard broadcasts and Berkeley broadcasts.

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Routers and IP Addressing Routers are also given IP addresses,

actually they are given 2 or more addresses since a router connects to more than one physical network.

To make network administrator’s lives easier, they often assign the same suffix to the various router addresses. (The prefixes must of course be different.)

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Multi-Homing

Sometimes computers also have multiple IP addresses (and multiple NIC cards).

If a host computer connects to multiple networks, it is called multi-homed.

This may increase reliability and performance, since it is still networked if only one of the networks goes down.

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References

Comer, Computer Networks and Internets

http://www.whatis.com http://www.webopdeia.com http://public.pacbell.net/dedicated/

cidr.html http://www.netsol.com/cgi-bin/whois/

whois