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3-2
Chapter 3 Internet Access Application
Note on IP Address Assignment
A unique 32-bit IP address is assigned to each host on the Internet. Similarly, every machine on
an internet must have a unique IP address. Do not assign an arbitrary address to any machine on
your network without prior consulting your network administrator.
IP Subnet Mask
A subnet mask is a 32-bit quantity that, when logically ANDed with an IP address, yields the
network number. For instance, the subnet masks for class A, B and C without subnetting are
255.0.0.0, 255.255.0.0 and 255.255.255.0, respectively.
The subnet mask is used to split the IP network addresses to create more network numbers. More
network numbers can be created by shifting some bits from the host ID to the network ID. For
instance, to partition a class C network number 192.68.135.0 into two, you shift 1 bit from the
host ID to the network ID. Thus the new subnet mask will be 255.255.255.128; the first subnet
will have network number 192.68.135.0 with hosts 192.68.135.1 to 192.68.135.126 and the
second subnet will have network number 192.68.135.128 with hosts 192.68.135.129 to
192.68.135.254.
It is recommended that you use the same subnet mask for all physical networks that share an IP
network number. Table 3-1 below lists the additional subnet mask bits in dot decimal notations.
To use Table 3-1, write down the original subnet mask and substitute the higher order 0s with the
dot decimal of the additional subnet bits. For instance, to partition your class C network
204.247.203.0 with subnet mask 255.255.255.0 into 16 subnets (4 bits), the new subnet mask
becomes 255.255.255.240.
Table 3-1. Subnet Mask Notation
Additional Subnet Mask Bits in Dot Decimal Notation
Number of Bits Dot Decimal Value
1
128
2
192