Would it be possible to configure a router for a 255.255.254.0 Ethernet and a 255.255.252.0 serial subnet? Does IGRP/RIPv1 support variable subnetting?

Yes it is possible to configure these subnet masks. In order to subnet on a Cisco router, the subnet bits must be contiguous, so 255.255.253.0 would not be valid (11111111.11111111.11111101.00000000) while 225.255.252.0 would be valid (11111111.11111111.11111100.00000000). Subnetting by borrowing all but one bits from the host portion is not allowed. También, traditionally, subnetting with a single bit was not allowed. The masks above satisfy these conditions. Refer to IP Addressing and Subnetting for New Usersfor more information.

IGRP RIP version 1 do not support variable length subnet masking (VLSM). A single router running any of these protocols would work fine with variable length subnetting. An incoming packet destined for one of the configured subnets would be routed properly and delivered to the correct destination interface. Sin embargo, if the VLSM and discontiguous networks are configured across multiple routers in IGRP domain, then it will lead to routing problems. Refer to Why Doesn’t RIP or IGRP Support Discontiguous Networks? for more information.

The newer IP routing protocols, EIGRP, EIIS, and OSPF, as well as RIP version 2, support VLSM, and they should be preferred in your network design. Refer to IP Routing Protocols Technical Support Pagefor more information on all IP routing Protocols.