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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.
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.
In previous versions of the document, [19] [20] network numbers were 8-bit numbers rather than the 32-bit numbers used in IPv4. At that time, three networks were added that were not listed earlier: 42.rrr.rrr.rrr, 43.rrr.rrr.rrr, and 44.rrr.rrr.rrr. The relevant portion of RFC 790 is reproduced here with minor changes:
In the above example, the subnet mask consists of 26 bits, making it 255.255.255.192, leaving 6 bits for the host identifier. This allows for 62 host combinations (2 6 −2). In general, the number of available hosts on a subnet is 2 h −2, where h is the number of bits used for the host
The result uses CIDR notation and can be used by an administrator to block a range of IP addresses. The template can be used by editing any page, inserting the template, and previewing the result. There is no need to save the edit.
Subnet Used for link-local addresses [5] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server 172.16.0.0/12 172.16.0.0–172.31.255.255 1 048 576: Private network Used for local communications within a private network [3] 192.0.0.0/24 192.0.0.0–192.0.0.255 256
Protocol Number Keyword Protocol References/RFC; 0x00 0 HOPOPT IPv6 Hop-by-Hop Option: RFC 8200: 0x01 1 ICMP Internet Control Message Protocol: RFC 792: 0x02 2 IGMP Internet Group Management Protocol: RFC 1112: 0x03 3 GGP Gateway-to-Gateway Protocol: RFC 823: 0x04 4 IP-in-IP IP in IP (encapsulation) RFC 2003: 0x05 5 ST Internet Stream Protocol ...