CIS307: IP Addresses

[Chapter 16 - Comer (1999)]

IP Addresses

IP Version 4 addresses, also called IPv4 addresses, or just IP addresses, are 32 bit integers. [IPv6, which is the new version of IP, and which we do not study, use an IP address with 128 bits.] They are normally written as 4 small integers representing the bytes of the number separated by periods. For example 155.247.182.1 is an IP address. Each IP address consists of two portions, a network identifier and a host identifier. There are 5 classes of IP addresses:

Class A: The network identifier is 1 byte and the host identifier is 3 bytes. The network identifier will start with a 0 bit. For example 126.46.31.87 is a class A address.
Class B: The network identifier is 2 bytes and the host identifier is 2 bytes. The network identifier will start with the bits 10. For example 155.247.170.2 is a class B address.
Class C: The network identifier is 3 bytes and the host identifier is 1 byte. The network identifier will start with the bits 110. For example 200.77.88.91 is a class C address.
Class D: It starts with the bits 1110 and it is used as a multicast address. For example 225.65.90.3 is a class D address.
Class E: It starts with bits 1111 and it is currently not in use.

A number of IP addresses have a standard meaning:

+------------+------------+----------+-------------------------------+
| Network    | Host       | Type of  | Purpose                       |
| Identifier | Identifier | Address  |                               |
+------------+------------+----------+-------------------------------+
| all 0s     |  all 0s    | this     | Used during bootstrap to      |
|            |            | computer | ask for own's IP address      |
+------------+------------+----------+-------------------------------+
| Network    |  all 0s    | specified| The specified network,        |
| Identifier |            | network  | independent of its hosts      |
+------------+------------+----------+-------------------------------+
| Network    |  all 1s    | specified| Broadcast address for the     |
| Identifier |            | network  | specified network.            |
+------------+------------+----------+-------------------------------+
| all 1s     |  all 1s    | local    | Broadcast on local network    |
|            |            | network  |                               |
+------------+------------+----------+-------------------------------+
| 127        | anything   | loopback | Testing of TCP/IP while not   |
|            |            |          | using the network             |
+------------+------------+----------+-------------------------------+

Subnetting

The granularity of IP address classes leads often to poor utilization of the address space and to limited ability to address subgroups within a network. The solution is to use Subnetting. Assume that we have a class B network like 155.247. We can partition the host space into 10 bits for subnet id and 6 bits for host id. Thus we have 1024 subnets each with up to 62 hosts (64 - 1 network - 1 broadcast). Subnetting is based on the use of masks. In our example, the subnet mask is 255.255.255.192. The bitwise AND of an IP address with the submask will result in the subnet identity. In our example, if we have the IP address 155.247.182.98, then the subnetwork id, also called extended-network-prefix, is 155.247.182.34 and the subnet is known as 155.247.182.34/26 to stress that it uses 26 bits, leaving the remaining 6 bits for the host-id. Notice that from far away packets will go to the network 155.247/16, and once there packets will go to the specific subnet, and from there to the intended host.

To account for subnetting a routing tables T takes the form:

[subnet-id, subnet-mask, next-hop] where the subnet-id is uniquely defined for a network (i.e. all the subnets of a network share the same mask, i.e. they have the same number of bits). The next-hop is the port of the router through which the current packet should be forwarded.

Then when an IP address A has to be routed the algorithm used is:

   For each row i of routing table T
       Let D = T[i].subnet-mask BitwiseAnd IP;
       If (D == T[i].subnet-id) then
       {
          Forward packet to T[i].next-hop;
          break;
       }

Normally, routing moves packets across the internet until the packet arrives to the destination network. Then the packet is directed to a specific host. With subnetting it becomes possible to route packets across the internet to arrive to a specific subnet, and then to move within the subnet to a specific host.

The ideas of masks and subnetting have been generalized to allow more complex partitions of networks than the one we have just discussed. In particular, variable length subnet masks have been used. This is done with the Classless Inter Domain Routing (CIDR). Now the masks used in routing can be of any size and in matching IP addresses one aims for the longest match. For example, suppose that in a routing table we have a row for the network 1101011110110 and a row for the network 11010111101 then, if we are looking for the destination 11010111110110111111001011101001 we will use the first row since it matches the given destination and it is more specific than the second row. CIDR helps reduce two kinds of problems: the fact that IP addresses are not efficiently allocated using the class oriented schema; and the fact that routing table may grow to be very large.

Routing Protocols, like RIP (Routing Information Protocol) and BGP (Border Gateway Protocol), are used by routers to exchange the information needed to maintain routing tables. Routing will be discussed in a later lecture. An Autonomous System is the technical term for somebody's network (or a group of related networks). The exchange of routing information within an autonomous system and among autonomous systems is done using a variety of gateway protocols. An autonomous system may be isolated and protected from its environment through the use of a firewall

ingargio@joda.cis.temple.edu