Introduction to TCP/IP
Most IP addresses today use IP version 4—we’ll
talk about IP version 6 later. IPv4 addresses are 32 bits long and are usually
written in “dot” notation. An example would be 126.96.36.199. The Internet
is actually a lot of small local networks connected together. Part of an IP address
identifies which local network, and part of an IP address identifies a specific
system or host on that local network. What part of an IP address is for the “network”
and what part is for the “host” is determined by the class or the
IP Addressing—Three Classes
- Class A: NET.HOST.HOST.HOST
- Class B: NET.NET.HOST.HOST
- Class C: NET.NET.NET.HOST
Before the introduction of subnet masks,
the only way to tell the network part of an IP address from
the host part was by its class.
Class A addresses have 8 bits (one octet) for the network
part and 24 bits for the host part. This allows for a small
number of large networks.
Class B addresses have 16 bits each for the network and host
Class C addresses have 24 bits for the network and 8 bits
for the host. This allows for a fairly large number of networks
with up to 254 systems on each.
IPv4 addresses are 32 bits with a network part and a host
Unless you are using subnets, you divide an IP address into
the network and host parts based on the address class.
The network part of an address is used for routing packets
over the Internet. The host part is used for final delivery
on the local net.
IP Addressing—Class A
Here’s an example of a class A address.
Any IPv4 address in which the first octet is less than 128
is by definition a class A address.
This address is for host #222.135.17 on network #10, although
the host is always referred to by its full address.
- Network # 10
- Host # 222.135.17
- Range of class A network IDs: 1–126
- Number of available hosts: 16,777,214
IP Addressing—Class B
Here’s an example of a class B address.
Any IPv4 address in which the first octet is between 128 and
191 is by definition a class B address
- Network # 128.128
- Host # 141.245
- Range of class B network IDs: 128.1–191.254
- Number of available hosts: 65,534
IP Addressing—Class C
Here’s an example of a class C address.
Most IPv4 addresses in which the first octet is 192 or higher
are class C addresses, but some of the higher ranges are reserved
for multicast applications.
-Network # 192.150.12
-Host # 1
-Range of class C network IDs: 192.0.1–223.255.254
-Number of available hosts: 254