All About IP Addresses

The key to understanding IP, and all of the issues related knowing what a routing table looks like and the effects each IP topic has in a routing table. To begin with, let's review the basics. IP addresses numbers, most commonly represented in dotted decimal notation (xxx.Each decimal number represents eight bits of binary data, and therefore decimal value between 0 and 255. IP addresses most commonly come or C. It's the value of the first number of the IP address that determines which a given IP address belongs. Class D addresses are used for multi applications.

(For a full explanation of class D addresses, refer to "Diving Through the Layers" .) The range of values for these classes are given below.

Class Range Allocation
A 1-126 N.H.H.H
B 128-191 N.N.H.H
C 192-223 N.N.N.H
D 224-239 Not applicable
N=Network
H=Host
Note 1: 127.0.0.0 is a class A network,
but is reserved for use as a loopback address
(typically 127.0.0.1).
Note 2: The 0.0.0.0 network is reserved for use as the default route.
Note 3: Class D addresses are used by groups of hosts or routers
that share a common characteristic: e.g. all OSPF devices
respond to packets sent to address 224.0.0.2
Note 4: Class E addresses exist (240-248),
but are reserved for future use

The class of an address defines which portion of the address identifies the Network number and which portion identifies the Host, as illustrated

So, without any subnetting (which we will come to a little later), a routing table will keep track of a) network numbers, b) the next hop router c) the interface this next hop router is reachable through. A simple network with the corresponding routing table for a Cisco router is



C 199.2.2.0 directly connected Ethernet 0
C 10.0.0.0 directly connected Token-ring 1
C 152.8.0.0 directly connected Ethernet 1
I 200.1.1.0 via 152.8.1.2 Ethernet 1

Since Cisco doesn't give headings for these columns, you need to know what each column consists of. The first column of the routing table number was discovered. C stands for Connected and I indicates the network was learned from the IGRP routing protocol. For a full description appears in a UNIX host and a Cisco router, refer to "Should RIP Rest In Peace" .
The important thing to realize is that while a routing table keeps track of network numbers, no one assigns a network number to any piece of router or host connected on the network must have an IP address and a subnet mask defined (many pieces of equipment will assign a default applied). From this IP address and subnet mask, the network number is derived by the IP stack and tracked in the routing table.
(This is the exact opposite of what happens in a NetWare network. In NetWare, you assign a network number to a server LAN card, which is wire. The workstations use MAC addresses as IPX node numbers.)
Routing tables can get very large. Internet backbone routers can have over 40,000 routes defined in them. In most corporate networks, the there are not so many subnets that need to be reached.
Many large routers, particulary internet routers, use a method called Classless Interdomain Routing (CIDR) to reduce the number of entries table. If we imagine, for instance, that all the Class C addresses that start with the value 194 are allocated for use in Europe, it would significantly entries in Internet routers in the US if there was only one entry for all these class C addresses, rather than a separate entry in the routing table (as in this example) all the networks with the first octet value of 194 are physically located in one area of the network.
IP addresses are used to deliver packets of data across a network and have what is termed end-to-end significance. This means that the source remains constant as the packet traverses a network. Each time a packet travels through a router, the router will reference it's routing table to number of the destination IP address with an entry in its routing table. If a match is found, the packet is forwarded to the next hop router for question (note that a router does not necessarily know the complete path from source to destination--it just knows the next hop router to go of two things happens. The packet may be forwarded to the router defined as the default gateway, or the packet may be dropped by the router. gateway is a router.)
Packets are forwarded to a default router in the belief that the default router has more network information in its routing table and will therefore correctly on to its final destination. This is typically used when connecting a LAN with PCs on it to the Internet. Each PC will have the router Internet defined as its default gateway.
A default gateway is seen in a routing table of a host as follows: the default route 0.0.0.0 will be listed as the destination network, and the IP will be listed as the next hop router.
If the source and destination IP addresses remain constant as the packet works its way through the network, how is the next hop router addressed? this is handled by the MAC (Media Access Control) address, as illustrated below. The key point is that the MAC addresses will change every router, however, the IP addresses will remain constant.



PC1 Router E0 Router E1 PC2
MAC Address M1 M2 M3 M4
Software (IP) address 11 12 13 14
A packet sent from PC1 to PC2 will look like this at point A:
Destination Source Destination Source Data
MAC MAC IP IP
M2 M1 14 11 1001001
A packet sent from PC1 to PC2 will look like this at point B:
Destination Source Destination Source Data
MAC MAC IP IP
M4 M3 14 11 1001001