/31 subnet

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.
trojan81Asked:
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JFrederick29Connect With a Mentor Commented:
/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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Ernie BeekExpertCommented:
/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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Ernie BeekExpertCommented:
Oh, /32 would define a single host, you're correct.
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Ernie BeekExpertCommented:
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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Ernie BeekExpertCommented:
Thx JFrederick29 :)
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trojan81Author Commented:
I gave it another thought and yes, they probably wanted to just save .80 from being used by something else.
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