Information Technology Reference
In-Depth Information
The ISO have adopted TCP/IP as the basis for the standards relating to the network and
transport layers of the OSI model. This standard is known as ISO-IP. Most currently avail-
able systems conform to the IP addressing standard.
Common applications that use TCP/IP communications are remote login and file transfer.
Typical programs used in file transfer and login over TCP communication are ftp for file
transfer program and telnet which allows remote log into another computer. The ping
program determines if a node is responding to TCP/IP communications.
23.2 TCP/IP gateways and hosts
TCP/IP hosts are nodes which communicate over interconnected networks using TCP/IP
communications. A TCP/IP gateway node connects one type of network to another. It con-
tains hardware to provide the physical link between the different networks and the hardware
and software to convert frames from one network to the other. Typically, it converts a Token
Ring MAC layer to an equivalent Ethernet MAC layer, and vice versa.
A router connects a network of a similar type to another of the same kind through a point-
to-point link. The main operational difference between a gateway, a router, and a bridge is
that for a Token Ring and Ethernet network, the bridge uses the 48-bit MAC address to route
frames, whereas the gateway and router use the IP network address. As an analogy to the
public telephone system, the MAC address would be equivalent to a randomly assigned tele-
phone number, whereas the IP address would contain the information on where the telephone
is logically located, such as which country, area code, and so on.
Figure 23.2 shows how a gateway (or router) routes information. It reads the frame from
the computer on network A, and reads the IP address contained in the frame and makes a
decision whether it is routed out of network A to network B. If it does then it relays the
frame to network B.
23.3 Function of the IP protocol
The main functions of the IP protocol are to:
Route IP data frames (which are called Internet datagrams) around an internet. The IP
protocol program running on each node knows the location of the gateway on the net-
work. The gateway must then be able to locate the interconnected network. Data then
passes from node to gateway through the Internet.
Fragment the data into smaller units, if it is greater than a given amount (64 kB).
Report errors - when a datagram is being routed or is being reassembled an error can
occur. If this happens then the node that detects the error reports back to the source node.
Datagrams are deleted from the network if they travel through the network for more than
a set time. Again, an error message is returned to the source node to inform it that the in-
ternet routing could not find a route for the datagram or that the destination node, or net-
work, does not exist.
 
Search WWH ::




Custom Search