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+--- Thread: is /25 in class C legal ? (/thread-418.html)
is /25 in class C legal ? - gtrunner - 06-09-2010
I'm getting mixed reports some books say its not legal (you cant use it ) and other places like a question on the site lets you use a /25 mask in vlsm. is it legal or not ?
Re: is /25 in class C legal ? - bryanrobh - 06-09-2010
A /25 is equivalent to 255.255.255.128. So if your using vlsm and you want to make a subnet for a network that needs no more than 126 hosts you can use /25. So yes it is legal.
Re: is /25 in class C legal ? - gtrunner - 06-10-2010
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q139983
Option 3 (unsupported)
Subnet mask of 255.255.255.128.
This gives you 2 subnets with a maximum of 126 clients on the Local LAN. Both subnets are technically invalid but work with Windows NT 3.5 and 3.51. Be sure all clients on the network support this subnet mask. Microsoft cannot guarantee this will work in future releases of Windows NT. Subnet 1 would have the fourth octet of 0-127 (1-126 can be assigned). Subnet 2 would have the fourth octet of 128-255 (129-254 can be assigned). For example XXX.XXX.XXX.1 is the first address of subnet 1 and could be used for the Windows NT network card and XXX.XXX.XXX.129 is the first address of subnet 2 and could be used for the RAS connection.
ok after an hour of looking at different sites its not supported by everything but all cisco routers past ISO versions 12.0 support it. If its not ver 12.0 you have to use IP subnet zero command. I've never seen this before