IPv6 networks and subnets

An IPv6 network is a group of hosts and routers that share a common network prefix and exist on the same VLAN interface. Where multiple unique network prefixes exist on the VLAN, each prefix corresponds to a different subnet. For example, if a given network has a prefix of 2001:db8:1ad:27b::/64, any global unicast address for an individual device belonging to this network has:

  • The same prefix (2001:db8:1ad:27b::/64).

  • A unique value (for the interface ID) in the remaining 64 bits of the global unicast addresses.

In the above case, if the device has an interface ID of 218:71ff:fedd:cf00, its complete global unicast address is: 

2001:db8:1ad:27b:218:71ff:fedd:cf00/64

Traffic between hosts on the same network is switched and employs link-local addresses that include a reserved prefix (FE80::/64) and a unique interface ID generated from the device MAC address. See IPv6 Addressing Configuration. Continuing the example from above, device "A" uses the following link-local address for switching:

However, when a packet must be sent from one network to another, where the source and destination have different IPv6 network prefixes, the packet must be routed. For example, routing is required to send traffic between the devices at these two addresses residing in different networks: 

2540:0:a03:e102:218:71ff:fedd:cf00/64
2001:0:db8:17fd:218:71ff:fedd:cf00/64