The subnet of this host.
Subnets can either be single MAC, IPv4 or IPv6 addresses, in which case a subnet consisting of only that single address is assumed, or they can be a IPv4 or IPv6 network address with a prefix length.
IPv4 subnets are notated like 192.168.1.0/24, IPv6 subnets are notated like fec0:0:0:1::/64. MAC addresses are notated like 0:1a:2b:3c:4d:5e.
Note that subnets like 192.168.1.1/24 are invalid.