• Topology: modem with router and Ethernet LAN bridge <---> server having one NIC.
  • Server network interface: It is capable of handling more than one IPv4 address. All addresses are within the same subnetwork.
  • Modem: Does the routing service and nothing else. Itself has one address for both WAN and LAN, also within the subnetwork.

Will there be an address conflict or will the ARP table have more than one IP, but always the same MAC address? We did encounter packet loss or host unreachability. The modem's ARP table did contain all addresses apart from its own, all associated with the same MAC.

1 Answer 1


Putting multiple IP addresses on a single interface (and thus a single MAC address) is quite common for servers.

For example: if a server has multiple roles and services running on it, it might be a good idea to give each role/service its own IP address. When you later want to move a role/service to a different machine you can do so without disrupting the other roles/services on the first server.

  • Thank you for your reply. I suppose neither packet loss nor unreachable hosts are related to addressing at all. In this particular case. Jan 4, 2014 at 10:28
  • 2
    You are correct. Jan 4, 2014 at 10:48
  • 3
    There are a growing number of seriously brain damaged "routers" on the market that have a flawed "arp cache" whereby it holds a one-to-one map of ip-to-mac -- one mac == one ip == one mac. This is the case on the 2wire devices on Uverse.
    – Ricky
    Jan 5, 2014 at 0:46
  • @Ricky, I have had an even dumber experience. It might be a DHCP thing, but... I put two NICs on my VPN-gateway server on my home network, and I configured the system to forward everything from one of the NICs to the VPN tunnel. I then made my MikroTik router route everything from a particular subnet via that specific NIC's IP. It didn't work. Turns out, it's been using one MAC for two IPs, and sending the packets for the VPN gateway to the wrong NIC. Morale of the story: when you're doing 2+ IPs on one device, check your ARP tables.
    – Nullcaller
    Sep 16, 2023 at 18:17
  • That's a host problem. Linux has the stupid idea it should answer an ARP for any interface seen on any interface. This is absolutely broken behavior that just confuses everyone, "just makes things work". (thus came the sysctl for arp filtering)
    – Ricky
    Sep 16, 2023 at 20:31

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