3Com LANPLEX 2500 EXTENDEDPLEX®2500 EXTENDED User Manual page 31

If layer 3 information is provided in the IP VLAN for which you are
configuring an IP interface, the subnet portion of both addresses must be
compatible.
For example:
IP VLAN subnet 157.103.54.0 with subnet mask of 255.255.255.0
IP host interface address 157.103.54.254 with subnet mask of
255.255.255.0
Layer 2 (bridging) communication is still possible within an IP VLAN (or
router interface) for the group of ports within that IP Interface's IP VLAN. IP
data destined for a different IP subnet uses the IP routing interface to get to
that different subnet, even if the destination subnet is on a shared port.
About VLANs 2-9