I am trying to implement BGP session. I was given this table.

When I try to configure loopbacks the first loopback works in the proper way

R1(config)#interface loopback 0
*Mar  1 00:45:15.839: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R1(config-if)#ip address

But when i try to configure loopback 1 that's the error I get

> R1(config)#interface loopback 1
> R1(config-if)#ip address
> Bad mask /24 for address

I don't know if i understand it in the proper way. Or maybe the table is wrong.

1 Answer 1


You can't have an IP address where the host part is all zeros (=network address) or all ones (=directed broadcast address). If you use any other address from to it'll work.

  • 1
    With a /32 mask Commented Dec 11, 2018 at 11:14
  • @BenjaminDale, or with a /31 address`
    – Ron Maupin
    Commented Dec 11, 2018 at 13:20
  • /31? interesting - what is the application? Commented Dec 12, 2018 at 0:09
  • @BenjaminDale /31 is used for numbered point-to-point links - essentially like /30 but with only half the address consumption.
    – Zac67
    Commented Dec 12, 2018 at 7:19
  • Sorry - I get what a /31 is - just wondering the application of putting a non /32 mask on a loopback interface would be? Commented Dec 13, 2018 at 0:54

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