routers and bridges for TCP/IP networking


Thread Starter

Andrey, Bob

Trying to find out the pro's and cons of routers and bridges that are used in TCP/IP networking.
Where can I get some good information comparing the two. Thanks to all.
TCP/IP routers read the I/P on incoming packets and use the address in conjunction with a routing table in order to decide whch network to use to send the packet out on.

Ethernet bridges remember which port an ethernet packet address arrives at, so when it is sending a packet to that adress it only sends to that port.

If your are interconnecting networks which operate under different TCP/IP domains you need a TCP/IP router.

If you have several ethernet segments that work under a single TCP/IP domain (the most common situation for in house networks), then using ethernet bridges is a simple config free way to considerably reduce network traffic without the hassle of managing multiple domains (allthough bridges work fine even when multiple intranet domains exist, ethernet is the 'outermost' packet). This is especially so if there is a lot of peer traffic.

It is a good point to raise on this list, a lot of people are doing control and supervision over the company ethernet. By having a switching hub or bridge between the factory segments and the office segments means that the office segments do not get cluttered up with lots of little supervision packets, and, more importantly (at least for us;-)) the factory side of the net does not get blasted with 100Mbyte+ print jobs when sales reps print full colour catalogues!

Using different TCP/IP domains for the SCADA is also a neat move, as it isolates the industrial net from office side network services, so you do not get tied into any re-hash the IT department decides to do.

Ralph Mackiewicz

> Trying to find out the pro's and cons of routers and bridges that are
> used in TCP/IP networking. Where can I get some good information
> comparing the two. Thanks to all.

If you are talking about a comparison for using routers versus bridges for the same application you should note that the two devices perform completely different tasks.

A router is used to interconnect indepenedent network segments into a larger complex network (like connecting your local network to the

A bridge is used to 1) convert one type of datalink/media to another (token ring <-> Ethernet) and/or 2) to filter datalink activity on
one physical segment from affecting other segments for which the activity is unnecessary. This latter functionality is commonly referred to as a "switch" (using modern terminology) if the media is Ethernet. Simplistically, the switch/bridge essentially connects wires together, not networks (using only switches you would still only have one network segment).

You can combine the routing and bridging functions into a single device called a 3-layer switch.

Routers are more complex because they must have knowledge of both nodes and routers. A bridge/switch only has to have knowledge of
nodes only. Routers process network address information and determine where messages need to be routed through a complex network while
switch/bridges only deal with the lowlevel address information contained on the Ethernet card.

You can do things with a router that you can't do with a switch (such as build a firewall that prevents IT from accessing IA networks) and
vice-a-versa (a switch can isolate segments for collision management).

A web search for "switch router tutorial" on any decent search engine will return lots of choices. Cisco Systems also has some pretty good
tutorials on its site:

ISA sponsors several courses on this subject which are very good at
explaining how the devices work for IA engineers.

Ralph Mackiewicz