Q: Read the following topic then explain what do you understand from it. LINK-LA
ID: 3602164 • Letter: Q
Question
Q: Read the following topic then explain what do you understand from it.
LINK-LAYER ADDRESSING
In an internetwork such as the Internet we cannot make a datagram reach its destination using only IP addresses. Each datagram, from the same source host to the same destination host, may take a different path. The source and destination IP addresses define the two ends but cannot define which links the packet should pass through. The IP addresses in a datagram should not be changed. If the destination IP address in a datagram changes, the packet never reaches its destination.
For that, we need another addressing mechanism:
The link-layer addresses (sometimes called a link address, sometimes a physical address and sometimes a MAC address).
When a datagram passes from the network layer to the data-link layer, the datagram will be encapsulated in a frame and two data-link addresses are added to the frame header. These two addresses are changed every time the frame moves from one link to another.
Therefore, Each frame carries the same datagram with the same source and destination IP addresses (N1 and N8), but the link-layer addresses of the frame change from link to link.
Explanation / Answer
Answer: In any kind of computer network, be it small like LAN or big like Internet, a given message to be sent from one source to destination is first broken into packets (datagrams). Destination address of a datagram are specified in network layer of the source and this datagram is delivered at the network layer of the destination. Source and destination addresses of a datagram cannot change.
But a datagram can take different routes to reach the destination from source. As long as destination address is intact in datagram, it will reach its destination whatever be the route it takes. A route is nothing but a series of links (of connecting nodes). Each link in the route is identified by an address called link address. Along a route, a datagram will be delivered across immediate link as determined by the link address. This link address is added to the datagram when it passes through data link layer (node). A datagram along with source and destination link addresses is called as a frame. Link addresses of a datagram are defined according to the route taken by the datagram.
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