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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 85
The Default subnet masks are:
• Class A – 11111111.00000000.00000000.00000000
255.0.0.0
• Class B – 11111111.11111111.00000000.00000000
255.255.0.0
• Class C – 1111111.11111111.11111111.00000000
255.255.255.0
Additional bits can be added to the default subnet mask for a
given Class to further subnet a network. When a bitwise
logical AND operation is performed between the subnet mask
and the IP address, the result defines the Subnet Address.
Some restrictions apply to subnet addresses. Addresses of all
“0”s and all “1”s are reserved for the local network (when a host
does not know it’s network address) and for all hosts on the
network (the broadcast address). This also applies to subnets.
A subnet address cannot be all “0”s or all “1”s. A 1-bit subnet
mask is also not allowed.
Calculating the Number of Subnets and Nodes
To calculate the number of subnets and nodes, use the formula
(2
n
– 2) where n = the number of bits in either the subnet mask
or the node portion of the IP address. Multiplying the number
of subnets by the number of nodes available per subnet gives
the total number of nodes for the entire network.
Example
00001010.00101010.01001001.11010010 10.42.73.210
Class A IP address
11111111.11100000.00000000.00000000 255.224.0.0
Subnet Mask
00001010.00100000.00000000.00000000 10.32.0.0
Network Address
00001010.00101010.11111111.11111111 10.32.255.255
Broadcast Address