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The routing prefix of an address is identified by the subnet mask, written in the same form used for IP addresses. For example, the subnet mask for a routing prefix that is composed of the most-significant 24 bits of an IPv4 address is written as 255.255.255.0.
A subnet mask is a bitmask that encodes the prefix length associated with an IPv4 address or network in quad-dotted notation: 32 bits, starting with a number of 1-bits equal to the prefix length, ending with 0-bits, and encoded in four-part dotted-decimal format: 255.255.255.0. A subnet mask encodes the same information as a prefix length but ...
In this case, the longest prefix of the candidate routes is 192.168.20.16/28, since its subnet mask (/28) is longer than the other entry's mask (/16), making the route more specific. Forwarding tables often contain a default route, which has the shortest possible prefix match, to fall back on in case matches with all other entries fail.
Since it would not be practical to manually provision networks at scale, in IPv6 networking, DHCPv6 prefix delegation (RFC 3633; RFC 8415 § 6.3) is used to assign a network address prefix and automate configuration and provisioning of the public routable addresses for the network. In the typical case of a home network, for example, the home ...
Prefix length: A matching route table entry with a longer subnet mask is always preferred as it specifies the destination more exactly. Metric: When comparing routes learned via the same routing protocol, a lower metric is preferred. Metrics cannot be compared between routes learned from different routing protocols.
The 128 highest addresses within each / 64 subnet prefix are reserved to be used as anycast addresses. [27] These addresses usually have the first 57 bits of the interface identifier set to 1, followed by the 7-bit anycast ID. Prefixes for the network can be of any length for routing purposes, but subnets are required to have a length of 64 bits.
IP addresses in dot-decimal notation are also presented in CIDR notation, in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask 255.0.0.0.
Static routing may have the following uses: When using static address configuration (in the absence of DHCP or Router Advertisements) it can be used to provide a default route, forming a special case of the longest prefix match as it has a prefix length of zero and therefore always matches, and always matches last.