All TCP/IP enabled devices connected to the
Internet have an Internet Protocol (IP) address. Just like a telephone number,
it helps to uniquely identify a user of the system. The Internet Assigned
Numbers Authority (IANA) is the organization responsible for assigning IP
addresses to Internet Service Providers (ISPs) and deciding which ones should
be used for the public Internet and which ones should be used on private
networks.
IP addresses are in reality a string of 32
binary digits or bits. For
ease of use, network engineers often divide these 32 bits into four sets of 8
bits (or octets), each representing a number from 0 to 255. Each number is then
separated by a period (.) to create the familiar dotted decimal notation. An example of
an IP address that follows these rules is 97.65.25.12.
IP uses a table for this task that associates networks
with the gateways by which they may be reached. A catch−all entry (the default
route) must generally be supplied too; this is the gateway associated with
network 0.0.0.0. All destination addresses match this route, since none of the
32 bits are required to match, and therefore packets to an unknown network are
sent through the default route.
IP
networks are structured similarly. The whole Internet consists of a number of
proper networks, called autonomous systems. Each system performs routing
between its member hosts internally so that the task of delivering a datagram
is reduced to finding a path to the destination host's network. As soon as the
datagram is handed to any host on that particular network, further processing
is done exclusively by the network itself.
A subnet takes responsibility for delivering datagrams to
a certain range of IP addresses. It is an extension of the concept of splitting
bit fields, as in the A, B, and C classes. However, the network part is now
extended to include some bits from the host part. The number of bits that are
interpreted as the subnet number is given by the so−called subnet mask,
or netmask. This is a 32−bit number too, which specifies the bit mask
for the network part of the IP address. Example :
class B network number of 149.76.0.0, and its netmask is
therefore 255.255.0.0.
Subnets are created to reflect existing boundaries, be
they physical (between two Ethernets), administrative (between two
departments), or geographical (between two locations), and authority over each
subnet is delegated to some contact person. However, this structure affects
only the network's internal behavior, and is completely invisible to the outside
world.
All other hosts can be accessed only through special−purpose
machines called gateways. A gateway is a host that is connected to two
or more physical networks simultaneously and is configured to switch packets
between them.
A subnetwork/ subnet is a
range of logical addresses within the address space that is assigned to an
organization. Subnetting is a hierarchical partitioning of the network address
space of an organization (and of the network nodes of an autonomous system)
into several subnets. Routers constitute borders between subnets. Communication
to and from a subnet is mediated by one specific port of one specific router,
at least momentarily.
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