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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.
Largest CIDR block (subnet mask) Host ID size Mask bits 100.64.0.0 – 100.127.255.255: ... Prefix/L Global ID (random) Subnet ID Number of addresses in subnet 48 bits
The term subnet mask is only used within IPv4. Both IP versions however use the CIDR concept and notation. In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively.
The route evaluation process in each router uses the longest prefix match method to obtain the most specific route. The network with the longest subnet mask or network prefix that matches the destination IP address is the next-hop network gateway. The process repeats until a packet is delivered to the destination host, or earlier along the ...
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.
When multiple route table entries match, the entry with the longest subnet mask is chosen as it is the most specific one. [2] If there are multiple routes with the same subnet mask, the route with the lowest metric is used. If there are multiple default routes, the metric is also used to determine which to use.