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Subnets allow the most significant bits of an IP address to be interpreted as routing information, and the least significant bits of an IP address to be interpreted as a unique node address on the local IP network. The local IP network (subnet) is the network that can be addressed without using a gateway or router. Traffic between subnets must pass through a router. Subnetting is supported by both IPv4 and IPv6 networks.


The number of bits that are interpreted as containing routing information are determined by the net mask. A bit being set in the netmask means that bit is interpreted as routing information. For example, if the IPv4 address is and the netmask is then the 10.134 provides the routing information and 134.10 provides the local address information. IP addresses that start 10.134 can be sent directly to their destination on the local network. IP addresses that start with any other numbers are not on the local network so must instead be sent to the router.

The netmask used in IPv6 is called the prefix length or CIDR notation which stands for Classless Inter-Domain Routing. In IPv6 the prefix length is represented by the number of bits that are used to identify the network portion of the address. The remaining bits are used to identify the hosts. For example, if the IPv6 address is 2001:0db8:2345:: and a prefix length is 64, the corresponding netmask is /64. This means the first 64 bits of IPv6 address are used to identify the network and the remaining 64 bits are used to identify hosts within the network.

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