Bridge - unites in itself the repeater and the analyzer of the staff of network data. The bridge takes the information from heading of the staff, analyzes MAC-ADDRESSES of a source and the addressee of each staff and defines accommodation of all network devices (from what party of the bridge they are) / Besides the bridge creates the table of MAC-ADDRESSES for each port. The table serves for management of the traffic of a network.

Switch - the device is a basis modern Fast Ethernet or Gigabit Ethernet networks. The principle of work of the switchboard consists in the following. On the basis of the MAC-ADDRESS of the addressee included in each package, the switchboard finds out, for whom the package received by it is intended. Then transfers this package to that port behind which there is an addressee. It the switchboard favourably differs from the concentrator transferring the received package all ports.

In switchboards there is an opportunity of the organization of simultaneous connections between any pairs ports of the device. Thus pairs ports (sender-addressee) form independent virtual channels. The switchboard is the trained device and keeps in memory the table of conformity between number of port and the MAC-ADDRESS of the device connected to them.

These tables can be static and dynamic. The static table is filled with the manager manually (only for operated switchboards). The dynamic table is filled with the switchboard during self-training.

In switchboards some methods of switching are applied:

• Store-and-forward switching
• Cut-through switching
• Fragment-Free switching
• Hybrid switching

Router - the multiport network device used in computer networks of data transmission. Router on the basis of the information on topology of a network (the table of routing) and the certain rules, makes of the decision on transfer of packages to their addressee. Usually device it is applied to communication of several segments of a network.

In a role of a router the workstation or a server can act. The server should have some network interfaces. Also the special software should be established. Routers of the top class is, as a rule, specialized devices. They unite in the separate case set routing modules.

The basic purpose of routers is a routing of the traffic of a network. Process of routing can be divided into two hierarchically connected levels:

• The Level of routing. At this level there is a work to the table of routing. The table of routing serves for definition of the address (a network level) a following router or directly addressee to an available address (a network level) and the addressee after definition of the address of transfer the certain target physical port of a router gets out. This process refers to as definition of a route of moving of a package. Adjustment of the table of routing is conducted by reports of routing. At the same level the list of necessary given services is defined;
• The Level of transfer of packages. Before transferring a package, it is necessary: to check up the control sum of heading of a package, to define the address (a channel level) the addressee of a package and to make directly sending a package in view of sequence, fragmentations, filtrations. These actions are carried out on the basis of the commands acting from a level of routing.

Definition of a route of data transmission occurs by program. Corresponding software carry names of reports of routing. The logic of their work is based on algorithms of routing. Algorithms of routing calculate cost of delivery and choose a way with smaller cost. The elementary algorithms of routing define a route on the basis of the least number of intermediate (transit) units on a way to the addressee. More complex algorithms in concept "cost" pawn some parameters. For example, a delay by transfer of packages, throughput of liaison channels or monetary cost of use of a liaison channel. The basic result of work of algorithm of routing is creation and support of the table of routing in which all routing information enters the name.

The table of routing contains the following information:

• The Valid addresses of devices in a network;
• The Service information of the report of routing;
• Addresses of the nearest routers.

The basic requirements shown to algorithm of routing are:

• The Optimality of a choice of a route;
• Simplicity of realization;
• Stability;
• Fast convergence;
• Flexibility of realization.

The optimality of a choice of a route is key parameter of algorithm of routing and a parameter of quality of work router.