What are the rules for a 128 subnet mask and useable subnets

rollingrelic
rollingrelic used Ask the Experts™
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What are the rules for a 128 subnet mask and useable subnets…Cisco seems to use ( 2n )for calculating the number of useable subnets…Microsoft   ( 2n – 2)  which would make a 128 mask invalid
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Commented:
If you have zero subnet enabled on Cisco you can use a 128 bit mask.

The standard is not to use the zero subnet.  That is why Microsoft uses 2n-2
Network Consultant
Commented:
The zero subnet stuff is old school stuff.  Every router maker on the market today supports zero subnets out of the box.

Commented:
But zero subnet is the reason Cisco is saying 2n whereas Microsoft is saying 2n-2, which is the question that was asked.
Been a while since my ccna days. If its a class c then its pointless. Class a and b could would make use of 128 mask
Ken BooneNetwork Consultant

Commented:
Right, I'm just saying microsoft is still referring to old school days and it really only matter that micorsoft says 2n-2 when you are taking their test ;)

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