TCP/IP Tutorial and Technical Overview - IBM Redbooks

Because of the all bits 0 and all bits 1 restrictions, this defines 2 18 -2 (from 1 to
262143) valid subnets. This split provides 262142 subnets each with a maximum
of 2 6 -2 (62) hosts.
The value applied to the subnet number takes the value of the full octet with
non-significant bits set to zero. For example, the hexadecimal value 01 in this
subnet mask assumes an 8-bit value 01000000. This provides a subnet value of
64.
Applying the 255.255.255.192 to the sample Class A address of 9.67.38.1
provides the following information:
00001001 01000011 00100110 00000001 = 9.67.38.1 (Class A address)
11111111 11111111 11111111 11------ 255.255.255.192 (subnet mask)
===================================== logical_AND
00001001 01000011 00100110 00------ = 9.67.38.0 (subnet base address)
This leaves a host address of:
-------- -------- -------- --000001 = 1 (host address)
IP will recognize all host addresses as being on the local network for which the
logical_AND operation described earlier produces the same result. This is
important for routing IP datagrams in subnet environments (refer to 3.1.3, “IP
routing” on page 77).
The subnet number is:
-------- 01000011 00100110 00------ = 68760 (subnet number)
This subnet number is a relative number. That is, it is the 68760th subnet of
network 9 with the given subnet mask. This number bears no resemblance to the
actual IP address that this host has been assigned (9.67.38.1). It has no
meaning in terms of IP routing.
The division of the original into is chosen by the
network administrator. The values of all zeroes and all ones in the field
are reserved.
Variable length subnetting example
Consider a corporation that has been assigned the Class C network
165.214.32.0. The corporation has the requirement to split this address range
into five separate networks each with the following number of hosts:
► Subnet 1: 50 hosts
► Subnet 2: 50 hosts
► Subnet 3: 50 hosts
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