Secondary addresses and multinetting are often confused with one another. Some vendors even have CLI commands for multinetting that use the term secondary address.
Multinetting
Multinetting refers to the configuration of multiple subnets on a given IP interface. In days prior to VLANs and the use of VLSM (variable length subnet mask) this was used as a way to configure multiple Class C subnets on the same IP interface so that one could have > 254 hosts reachable from that router port. This functionality continues to be used for legacy reasons, but in most modern networks multinetting has more or less been superceded by the use of classes addressing via VLSM and VLANs, each with its own subnet.
Secondary addresses
Secondary addresses refers to the configuration of multiple IP addresses from the same subnet on a given IP interface.
This is used by some standards-based protocols such as VRRP (RFC 5798) where we have a virtual IP and an interface IP address on a given interface. The virtual IP address is the one configured as the default gateway on hosts and the VRRP master responds to ARP requests for that address. On the other hand, the interface IP address typically is owned just by a given router and is used to reach the router when using protocols such as ping and ssh. If one wants the router to receive protocol packets on the virtual IP address, it needs to be configured as a secondary address. Additionally, a router will respond to any other addresses configured as secondary (that it doesn't own) when it becomes master.
In some legacy networks, hosts may be configured with different default gateway addresses within the same subnet. In that case, secondary addresses are required even without VRRP.
Multinetting
Multinetting refers to the configuration of multiple subnets on a given IP interface. In days prior to VLANs and the use of VLSM (variable length subnet mask) this was used as a way to configure multiple Class C subnets on the same IP interface so that one could have > 254 hosts reachable from that router port. This functionality continues to be used for legacy reasons, but in most modern networks multinetting has more or less been superceded by the use of classes addressing via VLSM and VLANs, each with its own subnet.
Secondary addresses
Secondary addresses refers to the configuration of multiple IP addresses from the same subnet on a given IP interface.
This is used by some standards-based protocols such as VRRP (RFC 5798) where we have a virtual IP and an interface IP address on a given interface. The virtual IP address is the one configured as the default gateway on hosts and the VRRP master responds to ARP requests for that address. On the other hand, the interface IP address typically is owned just by a given router and is used to reach the router when using protocols such as ping and ssh. If one wants the router to receive protocol packets on the virtual IP address, it needs to be configured as a secondary address. Additionally, a router will respond to any other addresses configured as secondary (that it doesn't own) when it becomes master.
In some legacy networks, hosts may be configured with different default gateway addresses within the same subnet. In that case, secondary addresses are required even without VRRP.
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