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/31 subnet

Posted on 2011-09-08
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Last Modified: 2012-05-12
Hi Experts,

Is there a valid need to have a /31 subnet?

For example, if dealing with an IPSEC VPN and one side specifies for the encryption domain 10.10.10.80/31, the only host IP useable would be 10.10.10.81. 10.10.10.80 would not be useable, am I correct?
So why not specify for the encryption domain 10.10.10.81/32.
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Question by:trojan81
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6 Comments
 
LVL 35

Expert Comment

by:Ernie Beek
ID: 36503447
/31 would be useless. The .80 would be the network address and the .81 the broadcast address. So no room for hosts.
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LVL 35

Expert Comment

by:Ernie Beek
ID: 36503462
Oh, /32 would define a single host, you're correct.
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LVL 43

Accepted Solution

by:
JFrederick29 earned 400 total points
ID: 36503480
/31 is just a way of covering the two hosts:

10.10.10.80
10.10.10.81

It isn't being used on an interface so the subnet/broadcast addresses aren't in play.  It simply saves one line of config (having to specify both hosts individually).

/32 would cover two extra hosts which is probably not the intention:

10.10.10.80
10.10.10.81
10.10.10.82
10.10.10.83

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LVL 35

Expert Comment

by:Ernie Beek
ID: 36503498
Yeah, sorry about that. Got mixed up with the translation :-~
In this matter it is a way of defining the hosts, so no bother with network/broadcast addresses.
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LVL 35

Expert Comment

by:Ernie Beek
ID: 36503526
Thx JFrederick29 :)
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Author Closing Comment

by:trojan81
ID: 36505484
I gave it another thought and yes, they probably wanted to just save .80 from being used by something else.
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