subnetting

cb3dwa

im pritty good at subnetting if i do say so myself but ive just come across this question and dont understand how the answer is correct?

Which class of network license allows the borrowing of 15 bit to create subnets?

answer: Class A

Read more: Which class of network license allows the borrowing of 15 bit to create subnets

could some one explain please

fsanyee

A: 24 host bit : 24-15 = /23 mask
B: 16 host bit : 16 -15 not ok, because you can't do too much thing with a /31 subnet
C: 8 host bit : not ok.

SharkDiver

fsanyee is absolutely correct, I just want to elaborate.

In a class C, the host portion is only 8 bits to begin with. The most bits that you can use for subnetting is 6, which would put you at a /30 mask (255.255.255.252)

In a class B, the host position is 16 bits, but the most bits you can use for subnetting is 14, which would once again put you at a /30 mask

In a class A, you have 24 bits for the host portion and you can easily borrow 15 of those which would put you at a /23 mask (255.255.254.0)

As stated above, a /31 mask is useless as you would only have 2 IPs, and the first and last can't be used, which would leave you with zero.

Roguetadhg

SharkDiver wrote: »

fsanyee is absolutely correct, I just want to elaborate.

In a class C, the host portion is only 8 bits to begin with. The most bits that you can use for subnetting is 6, which would put you at a /30 mask (255.255.255.252)

In a class B, the host position is 16 bits, but the most bits you can use for subnetting is 14, which would once again put you at a /30 mask

In a class A, you have 24 bits for the host portion and you can easily borrow 15 of those which would put you at a /23 mask (255.255.254.0)

As stated above, a /31 mask is useless as you would only have 2 IPs, and the first and last can't be used, which would leave you with zero.

I heard, at one point you could use the 31 address as a point to point subnet (between 2 routers).

http://packetlife.net/blog/2008/jun/18/using-31-bit-subnets-on-point-point-links/

a quick google search

Cisco's site brought this up: http://www.cisco.com/en/US/docs/ios/12_2t/12_2t2/feature/guide/ft31addr.html

"Using a 31-bit prefix length leaves only two numbering possibilities, 0 and 1. In a point-to-point link with a 31-bit subnet mask, these two addresses must be interpreted as host addresses, and directed broadcast to the link will be eliminated. Limited broadcast must be used for all broadcast traffic on a point-to-point link with a 31-bit mask assigned to it."

SharkDiver

Rogue,

Good catch.

I just tried it on a real 2911 router and got this warning.

% Warning: use /31 mask on non point-to-point interface cautiously

But, it will work.
Nice!