Destination and Next Hop Addresses

What is the relationship between the destination address in a datagram header and the address of the next hop to which the datagram is forwarded?The Destination IP address field in a datagram contains the address of the Ultimate destination.When a router receives a datagram,the router extracts the destination address D,and uses it to compute the address of the next router to which datagram should be sent,N.Although the datagram is sent directly to address N,the header in the datagram retains destination address D.All routes are computed using IP address.After computing the address of a next hop,N,IP software uses the address binding to translate N to an equivalent hardware address for transmission.

The Berkeley Broadcast Address Form

The University of California at Berkeley developed and distributed an early implementation of TCP/IP protocols as a part of BSD UNIX.The BSD implementation contained a non-standard feature that has affected many subsequent implementations.Instead of using a host suffix of all ones to represent a directed broadcast address,the Berkeley implementation used a host suffix that contained all zeros.The address form is informally known as Berkeley Broadcast.Unfortunately,many computer manufacturers derived their early TCP/IP software from the Berkeley implementation,and a few sites still use Berkeley broadcast.Some implementations include a configuration parameter that can select between the TCP/IPTCP/IP standard and Berkeley form;many implementations are built to accept both standard and Berkeley broadcast address forms.

Directed Broadcast Address

Sometimes,it is convenient to send a copy of a packet to all hosts on a physical network.To make broadcast easy,IP defines a directed broadcast address for each physical network.When a packet is sent to network's broadcast address,a single copy of the packet travels across the internet until it reaches the specified network.The packet is then delivered to all the hosts on the network.The directed broadcast address for a network is formed by adding a suffix that consists of all 1 bits to the network prefix.

To ensure that each network can have directed broadcast address,IP reserves the host address that contains all 1 bits.An administrator must not assign the all ones host address to a specific computer or the software may malfunction.If a network hardware supports broadcast,a directed broadcast will be delivered using the hardware broadcast capability.In such cases,transmission of the packet will reach all computers on the network.When a directed broadcast is sent to a network that does not have hardware support for broadcast,software must send a separate copy of the packet to each host on the network.

Authority for Addresses

Throughout an internet,each network prefix must be unique.For networks connected to the global internet,an organization obtains network numbers from the communication company that supplies internet connections.Such companies are called Internet Service Providers(ISPs).Internet Service Providers coordinate with a central organization,the Internet Assigned Number Authority,to ensure that each network prefix is unique throughout the entire Internet.For a private internet,the choice of the network prefix can be made by organization.To ensure that each prefix is unique, group that builds a private internet must decide how to coordinate network number assignments.Often,a single network administrator assign prefixes to all networks in the company's internet to ensure that numbers are not duplicated.

Loopback Address

IP defines a loopback address used to test network applications.Programmers often use loopback testing for preliminary debugging after a network application has been created.To perform a loopback test,a programmer must have two application programs that are intended to communicate across a network.Each application includes the code needed to interact with TCP/IP protocol software.Instead of executing each program on a separate computer,the programmer runs both program on a single computer and instructs them to use a loopback IP address when communicating.

When one application sends data to another,data travels from the protocol stack to the IP software,which forwards it back up through the protocol stack to the second program.Thus,the programmer can test the program logic quickly without needing two computers and without sending packets across a network.IP reserves the network prefix 127/8 for use with loopback.The host address used with 127 is irrelevant that means all host addresses are treated the same.By convention.programmers often use host number 1,making 127.0.0.1 the most popular form of loopback.During loopback testing no packets ever leave a computer that means the IP software forwards packets from one application program to another.Consequently,the loopback address never appears in packet traveling across a network.

Subnet and Classless Addressing

As the Internet grew,the original classful addressing scheme became a limitation.On one hand,the IP address space was being exhausted.On the other hand,because all networks had to choose one of the three possible sizes,that means many addresses were unused.Two new mechanisms were invented to overcome the limitations,known as subnet addressing and classless addressing.These two addressing schemes are so closely related that they can be considered to be a part of single abstraction.The generalization is straight forward that means instead of having three distinct address classes,allow the division between prefix and suffix to occur on an arbitrary bit boundary.Let understand this situation with the help of an example,Consider a network that contains 9 hosts.Only four bits of host suffix are needed to represent all possible host values.However,a class C address,which has the fewest address hosts possible,devote eight bits to the host suffix.Classless addressing solves the problem by allowing an ISP(Internet Service Provider) to assign a prefix that is 28 bits long that means network can have up to 14 hosts.