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A subnet mask encodes the same information as a prefix length but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous. Subnet masks were allowed by RFC 950 [6] to specify non-contiguous bits until RFC 4632 [5]: Section 5.1 stated that the mask must be left contiguous. Given this constraint, a subnet mask and ...
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.
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
A subnetwork, or subnet, is a logical subdivision of an IP network. [ 1 ] : 1, 16 The practice of dividing a network into two or more networks is called subnetting . Computers that belong to the same subnet are addressed with an identical group of its most-significant bits of their IP addresses .
For example, the U flag indicates that an IP route is up. filtering criteria: Access-control lists associated with the route; interface: Such as eth0 for the first Ethernet card, eth1 for the second Ethernet card, etc. Shown below is an example of what the table above could look like on a computer connected to the internet via a home router:
This means that 256 /8 address blocks fit into the entire IPv4 space. As IPv4 address exhaustion has advanced to its final stages, some organizations, such as Stanford University , formerly using 36.0.0.0 / 8 , have returned their allocated blocks (in this case to APNIC ) to assist in the delay of the exhaustion date.
Previously, every link needed to dedicate a / 31 or / 30 subnet using 2 or 4 IP addresses per point-to-point link. When a link is unnumbered, a router-id is used, a single IP address borrowed from a defined (normally a loopback) interface. The same router-id can be used on multiple interfaces.
A classful network is an obsolete network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing (CIDR) in 1993. The method divides the IP address space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits.