Network 10.1.1.0/31

[wfqk]

Hi Network 10.1.1.0/31 includes two routes 10.1.1.0/32 and 10.1.1.1/32

People often use these two in production. I remember the first one is subnet and second one is broadcast, and usually the two cannot be used. How to explain it? Thank you

[Dennis Mink]

yeah those are the basic of IP addresses , all 0's in their subnet (based on mask) are network addresses and all 1's are broadcasts. you just cant use them.

[wfqk]

Thank you for your reply. so if we have 10.1.1.0/30 in point to point link, do you think 10.1.1.0/30 and 10.1.1.3/30 useful? 

[Dennis Mink]

No, not on a point to point link, because there is no point to do a broadcast on a link with only 2 endpoints.

please rate if useful

[Georg Pauwen]

Hello,

/31 bit subnets don't have a network and broadcast address, and are typically used on point to point links where both network and broadcast addresses are not needed.

So, 10.1.1.0/31 and 10.1.1.1/31 are actually usable addresses. ISPs use /31 bit masks a lot to conserve IP addresses...

[Karsten Iwen]

In addition to what Georg explained, you can read more about it in RFC 3021 where this is defined.

[Joseph W. Doherty]

Hi Network 10.1.1.0/31 includes two routes 10.1.1.0/32 and 10.1.1.1/32

As host routes, but generally routing protocols that support /31s just carry the /31.

Oh, we also use /31s in our network for p-2-p use, and have lots of them - again with a routing protocol that supports, they work just fine.