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,
10.0.0.0specifies 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
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 (2n – 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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