When would you choose to use the
ip default-network command instead of the
ip default-gateway command?
ip default-gateway is to be used in L2 devices.
ip default-network would be used in L3 devices, but works slightly different from the usual static default route configured with
ip route 0.0.0.0 0.0.0.0 <next_hop>. Let's say you configure
ip default-network 172.31.0.0. Then, if the device already knows a route for 172.31.0.0, this route will be flagged as a default route candidate.
You need to take into account that
ip default-network command is classful.
Further details and clarifications are available here
ip default-gateway is for layer-2 devices and
ip default-network is for layer-3 devices.
ip default-network is basically a default route
When IP routing is enabled, the commands ip default-network and "ip route 0.0.0.0" etc. work the same. These commands would be used on a router or Layer 3 switch.
"ip default-gateway" would be used when ip routing is not enabled - for instance, on a Layer 2 switch that has an SVI for management but cannot route on that SVI.