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Can a router be connected to both a subnet 1 and a subnet 1.1 which is a subnet inside of subnet 1?

For example, can a router be connected and sent packets to subnet 223.1.17.128/25 and 223.1.17.192/28?

Thanks.

Caleb Bolton
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1 Answers1

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Yep, it's possible. But the packets will be routed through only one route with longest prefix match.

Let's guess you have router with three interfaces:

  • e0 - uplink - address 192.0.2.10/24, default gateway - 192.0.2.1
  • e1 - lan1 - 223.1.17.129/25
  • e2 - lan2 - 223.1.17.193/28

And routing table will be seems like:

static 0.0.0.0 0.0.0.0 via 192.0.2.1 dev e0
connected 192.0.2.0/24 dev e0
connected 223.1.17.128/25 dev e1
connected 223.1.17.192/28 dev e2

Simplify the corner cases like routing to local addresses and broadcast addresses, we get following behaviour for overlapped address spaces:

  • The packets to addresses 223.1.17.192-223.1.17.207 will be routed through e2 interface.
  • The packets to addresses 223.1.17.128-233.1.17.191 and 223.1.17.208-223.1.17.255 will be routed through e1 interface.

Thus, the hosts with addresses from range 223.1.17.192-223.1.17.207, connected to e1 interface (that has /25 prefix) will be in the blind spot. They won't have access through the router, only to hosts in same broadcast domain. To avoid this negative behaviour you should use something like vrf on the router.

Anton Danilov
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