<P> An address fulfills the functions of identifying the host and locating it on the network . The most common network addressing architecture is Internet Protocol version 4 (IPv4), but its successor, IPv6, has been increasingly deployed since approximately 2006 . An IPv4 address consists of 32 bits, for readability written in a form consisting of four decimal octets separated by dots, called dot - decimal notation . An IPv6 address consists of 128 bits written in a hexadecimal notation and groupings of 16 bits, called hextets, separated by colons . An IP address is divided into two logical parts, the network prefix and the host identifier . All hosts on a subnetwork have the same network prefix . This prefix occupies the most - significant bits of the address . The number of bits allocated within a network to the prefix may vary between subnets, depending on the network architecture . The host identifier is a unique local identification and is either a host number on the local network or an interface identifier . </P> <P> This addressing structure permits the selective routing of IP packets across multiple networks via special gateway computers, called routers, to a destination host if the network prefixes of origination and destination hosts differ, or sent directly to a target host on the local network if they are the same . Routers constitute logical or physical borders between the subnets, and manage traffic between them . Each subnet is served by a designated default router, but may consist internally of multiple physical Ethernet segments interconnected by network switches . </P> <P> 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 . </P> <P> The modern standard form of specification of the network prefix is CIDR notation, used for both IPv4 and IPv6 . It counts the number of bits in the prefix and appends that number to the address after a slash (/) character separator . This notation was introduced with Classless Inter-Domain Routing (CIDR). In IPv6 this is the only standards - based form to denote network or routing prefixes . </P>

When is a subnet mask required under tcp/ip