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Understanding Network/Subnet

I recently got a new customer whose network contains a few vlans.
This is an example from the router configuration:

Interface: eth0.20
IP Adresse: 172.17.20.1/24

Interface: eth0.21
Ip adress: 172.17.73.251/24
172.17.73.250/32
172.17.73.253/24

I would like to understand why the network at interface eth0. 21 has three ip addresses. Can someone explain that ?

regards

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Kimputer

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Steve

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Soulja

This is most likley done from exhausting their address space, so they took the lazy route and added secondary ip addresses on the same sub interface to allott for more ip addresses without having to create another vlan.

Soulja

One way to check if this is the case is to check the arp table on that router.

deibel

ASKER

there are some ip adresses an all have different mac address.
the ip adress listet above doesent apear in the arp list.
the ip adress 172.17.73.252 is lised with mac address

atlas_shuddered

There are numerous reasons why this may be set up like this. Source routing, conditional traffic forwarding, gateway filtering, etc. Be a lot easier to pin it down with the full config

skullnobrains

a likely guess based on usage and network masks
251 is the router's real address
252 would be the secondary router
253 is a carp/vrrp shared address between the 2 routers used as the gateway for the VLAN
250 is an ip used to redirect a single service to a separate host through port redirection which is likely a shared address as well.