Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member


Hi all, if my isp routes say a /28 block to me, and I put a /24 subnet mask on my interface, but with the correct ip assigned, what will happen ?


Re: routing

Depends. If your ISP has proxy-arp enabled you might be able to connect to things in the /24 subnet, but chances are they have already allocated those addresses, so they'll start seeing duplicate IP messages and they'll know something is up.

If they don't have proxy arp enabled, you run the risk of not being able to access everything in your subnet. You should probably match the config your ISP is giving you.

Hall of Fame Super Silver

Re: routing

Hello Carl,

I suppose you are talking of ip addresses given to your company not of the direct link between your router and the ISP router.

I think the ISP will advertise only the /28 block and will be not aware of your systems/servers out of that block and they will be unreachable from internet.

For example ip address is reserved for you and you allocate a /24 addresses after will be useless for your company.

Your ISP will route packets for to another customer or simply they will discard it.

If you need more then a /28 think of using NAT overload on your network.

hope to help


New Member

Re: routing

hi there, if I put my pc on an ip in that range, but with a /24 mask, will it work ?

Cisco Employee

Re: routing


It will work but chances are your ISP will assigned other /28 from that same /24 to other customers. It will definitely be an issue if you ever need connectivity to any of the other /28 subnets, as your router will consider these addresses as local because of the /24 configured on the interface of your router.


Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
CreatePlease to create content