Learn Windows XP Professional: TCP/IP Addressing in Windows XP Professional (Page 3) Page 3

Subnet Masks - Implementing subnewtorks (commonly referred to as subnets in the field) helps to control network traffic. Every node on the same physical Ethernet network sees all the packets of data sent out on the network. This commonly has the result of multiple collisions causing network performance to be slow. Routers or gateways are used to separate networks into subnets. Subnet masks on each of the nodes allow the nodes on the same subnetwork to continue to communicate with one another and with the routers or gateways they use to send their messages.

Subnet masks allow you to identify the network ID and the host (node) ID of an IP address.

Given the following example of a default B Class subnet mask:


we can determine that the network ID is 158.21 and the host ID is 57.111

Network Address :

Subnet Address :
Subnet Mask :
Subnet bit mask : nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Subnet Bits : 16
Host Bits : 16
Possible Number of Subnets : 1
Hosts per Subnet : 65534

Additional bits can be added to the subnet mask for a given class of addresses to subnet networks further.

Given the following example of a B Class address using an additional bit subnet mask:

11111111.11111111.11110000.00000000 Subnet Mask
10010110.11010111.00010000.00000000 Network address

Subnet Mask :
Subnet bit mask : nnnnnnnn.nnnnnnnn.nnnnhhhh.hhhhhhhh
Subnet Bits : 20
Host Bits : 12
Possible Number of Subnets : 16
Hosts per Subnet : 4094

we can see that rather than having the single subnet and 65534 Hosts per Subnet allowed under the default subnet mask, we are able to have up to 16 subnets with up to 4094 Hosts per Subnet by using a Subnet Mask of

Selected Subnet :
Usable Addresses : 4094
Host range : to
Broadcast :

Subnet Mask Subnet  Host Range Broadcast 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to 4094  to

[NOTES FROM THE FIELD] - A subnet address cannot be all 0's or all 1's.

This article was originally published on Aug 5, 2002

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