1
GB
Appendix
Netmask
The partition scheme that separates the different address classes is used to identify a net-
mask attached to the IP address. A netmask is a 32-bit quantity that, in logical combination
(using an AND operator) with an IP address, forms the network address. The netmasks for
Class A, B and C are for instance 255.0.0.0, 255.255.0.0 and 255.255.255.0.
The address 192.168.170.237 is a Class C IP address; its network portion is 192.168.170.
Combining it with the Class C netmask using an AND operator as shown in this example
leaves only the network portion of the address:
11000000 10101000 10101010 11101101 (192.168.170.237)
combined with:
11111111 11111111 11111111 00000000 (255.255.255.0)
resulting in:
11000000 10101000 10101010 00000000 (192.168.170.0)
An alternative to dotted decimal representation is to express the netmask as the quantity
of ones from left to right. This number is added to the IP address after a slash (/) as '/n'.
In this example, the address 192.168.170.237/24 could be written. This indicates that the
netmask is 24 ones followed by eight zeroes.
Subnet Addressing
You can see that even with a Class C address, there are a large number of hosts per network.
It would be a very inefficient use of the amount of IP addresses to supply every LAN with
so many IP addresses. A smaller office LANs does not have that many devices. A more
efficient technique is known as subnet addressing.
Subnet addressing allows us to split one IP network address into smaller multiple physical
networks known as subnetworks. Some of the node numbers are used as a subnet number
instead. A Class B address gives us 16 bits of node numbers translating to 64,000 nodes.
Most organizations do not use 64,000 nodes, so there are free bits that can be reassigned.
Subnet addressing makes use of those bits that are free.
A Class B address can be effectively translated into multiple Class C addresses. For exam-
ple, the IP address of 172.16.97.235 would be interpreted as IP network address 172.16,
subnet number 97, and node number 235. In addition to extending the number of ad-
dresses available, subnet addressing provides other benefits. Subnet addressing allows
a network manager to construct an address scheme for the network by using different
subnets for other geographical locations in the network or for other departments in the
organization.
