I think the idea of one interface per subnet originates in the early RFCs, such as RFC 1009 "Requirements for Internet Gateways": "Section 1.1.2 Networks and Gateways ... A gateway is connected to two or more networks, appearing to each of these networks as a connected host. Thus, it has a physical interface and an IP address on each of the connected networks ... " So by using singular terminology ( "a connected host", "a physical interface", "an IP address") instead of plural, a single interface per subnet for gateways (read routers) is implied. This is not to say that it will not work, at least on servers. Standards aside, a good reason why this is not a best practice is the concept of asynchronous routing where a packet arrives on one interface, and the reply leaves on the other interface with a different source IP on the reply. Most firewalls will reject packets such as this. -----Original Message----- From: Chris Meidinger [mailto:cmeidinger@sendmail.com] Sent: Monday, May 11, 2009 1:29 PM To: nanog@nanog.org Subject: two interfaces one subnet Hi, This is a pretty moronic question, but I've been searching RFC's on- and-off for a couple of weeks and can't find an answer. So I'm hoping someone here will know it offhand. I've been looking through RFC's trying to find a clear statement that having two interfaces in the same subnet does not work, but can't find it that statement anywhere. The OS in this case is Linux. I know it can be done with clever routing and prioritization and such, but this has to do with vanilla config, just setting up two interfaces in one network. I would be grateful for a pointer to such an RFC statement, assuming it exists. Thanks! Chris