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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Class D addresses are reserved for multicasting.
Class E Addresses are reserved for future use. They are not used for node addresses.
The part of the IP address that belongs to the network is the part that is ‘hidden’ by the ‘1’s in the
subnet mask. This can be seen below:
• Class A NETWORK.node.node.node
• Class B NETWORK.NETWORK.node.node
• Class C NETWORK.NETWORK.NETWORK.node
For example, the IP address 10.42.73.210 is a Class A address, so the Network part of the address
(called the Network Address) is the first octet (10.x.x.x). The node part of the address is the last three
octets (x.42.73.210).
To specify the network address for a given IP address, the node part is set to all “0”s. In our example,
10.0.0.0 specifies the network address for 10.42.73.210. When the node part is set to all “1”s, the
address specifies a broadcast address. So, 10.255.255.255 is the broadcast address for the network
10.0.0.0.
Subnet Masking
A subnet mask can be applied to an IP address to identify the network and the node parts of the
address. A bitwise logical AND operation between the IP address and the subnet mask results in the
Network Address.
For example:
00001010.00101010.01001001.11010010 10.42.73.210 Class A IP address
11111111.00000000.00000000.00000000 255.0.0.0 Class A Subnet Mask
00001010.00000000.00000000.00000000 10.0.0.0 Network Address
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
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