network layer is concerned with getting packets from the source all the way to
the destination. The packets may require to make many hops at the intermediate
routers while reaching the destination. This is the lowest layer that deals
with end to end transmission. In order to achieve its goals, the network layer
must know about the topology of the communication network. It must also take
care to choose routes to avoid overloading of some of the communication lines
while leaving others idle. The network layer-transport layer interface
frequently is the interface between the carrier and the customer, that is the
boundary of the subnet. The functions of this layer include :
1. Routing -
The process of transferring packets received from the Data Link Layer of the
source network to the Data Link Layer of the correct destination network is called
routing. Involves decision making at each intermediate node on where to send
the packet next so that it eventually reaches its destination. The node which
makes this choice is called a router. For routing we require some mode of
addressing which is recognized by the Network Layer. This addressing is
different from the MAC layer addressing.
- The network layer is the same across all physical networks (such as
Token-Ring and Ethernet). Thus, if two physically different networks have to communicate,
the packets that arrive at the Data Link Layer of the node which connects these
two physically different networks, would be stripped of their headers and
passed to the Network Layer. The network layer would then pass this data to the
Data Link Layer of the other physical network.
Control - If the incoming rate of the packets arriving at any router is more
than the outgoing rate, then congestion is said to occur. Congestion may be
caused by many factors. If suddenly, packets begin arriving on many input lines
and all need the same output line, then a queue will build up. If there is
insufficient memory to hold all of them, packets will be lost. But even if
routers have an infinite amount of memory, congestion gets worse, because by the
time packets reach to the front of the queue,
already timed out (repeatedly), and duplicates have been sent. All these
packets are dutifully forwarded to the next router, increasing the load all the
way to the destination. Another reason for congestion are slow processors. If
the router's CPUs are slow at performing the bookkeeping tasks required of
them, queues can build up, even though there is excess line capacity.
Similarly, low-bandwidth lines can also cause congestion.
now look at these function one by one.
addresses are of 4 bytes and consist of :
network address, followed by
ii) The host
part identifies a network on which the host resides and the second part
identifies the particular host on the given network. Some nodes which have more
than one interface to a network must be assigned separate internet addresses
for each interface. This multi-layer addressing makes it easier to find and
deliver data to the destination. A fixed size for each of these would lead to
wastage or under-usage that is either there will be too many network addresses
and few hosts in each (which causes problems for routers who route based on the
network address) or there will be very few network addresses and lots of hosts
(which will be a waste for small network requirements). Thus, we do away with
any notion of fixed sizes for the network and host addresses.
classify networks as follows:
1. Large Networks: 8-bit
network address and 24-bit host address. There are approximately 16 million hosts per network and a maximum of 126 (
2^7 - 2 ) Class A networks can be defined. The calculation requires that 2 be
subtracted because 0.0.0.0 is reserved for use as the default route and
127.0.0.0 be reserved for the loop back function. Moreover each Class A network
can support a maximum of 16,777,214 (2^24 - 2) hosts per network. The host
calculation requires that 2 be subtracted because all 0's are reserved to
identify the network itself and all 1s are reserved for broadcast addresses.
The reserved numbers may not be assigned to individual hosts.
2. Medium Networks: 16-bit network address and 16-bit
host address. There are approximately
65000 hosts per network and a maximum of 16,384 (2^14) Class B networks can be
defined with up to (2^16-2) hosts per network.
3. Small Networks: 24-bit network address and 8-bit
host address. There are approximately 250
hosts per network.
think that Large and Medium networks are sort of a waste as few corporations or
organizations are large enough to have 65000 different hosts. (By the way,
there are very few corporations in the world with even close to 65000
employees, and even in these corporations it is highly unlikely that each
employee has his/her own computer connected to the network.) Well, if you think
so, you're right. This decision seems to have been a mistake.
specifications divide addresses into the following classes :