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Does/Can network bridges bridge 2 different network segments (with 2 different network/subnet mask) ? Or all the segments must be long to the same subnet in order to communicate ?
What are the reasons for using a network bridge ?
Regards, Noob
Noob
Posted 2015-05-07T09:26:29.970
Reputation: 1 145
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Let's start from the basic:
A bridge is unaware of IP because it works at layer 2 (ethernet).
A Single LAN is a single broadcast domain and a single collision domain. It means that when an equipment send a broadcast frame everyone can hear it and when a collision happens everyone can hear it.
When you use a bridge to connect two segments you have a single broadcast domain and two collision domains. It means that a broadcast frame will pass through the bridge from one segment to the other and everyone can hear it, but a collsion happening on one segment won't be heard on the other.
As more equipments you have in a single LAN, more collisions happen and it slows the LAN. A bridge is a very good way to segment a crowded LAN to reduce the number of collsions. The ultimate bridge is a switch, because a switch is a multiport bridge where each port is a segment.
Now let's go to layer 3.
IP is a layer 3 protocol (remember that the bridge is unaware of it). If on each segment you have a different IP network they won't be able to communicate. You will need a router or a layer 3 switch to interconnect the two segments at IP level.
jcbermu
Posted 2015-05-07T09:26:29.970
Reputation: 15 868
Hi jcbermu, if bridge is a layer 2 device unaware of L3. Why can't we send frames across it from devices in different subnet. Since the layer 2 bridge/switch will not even check it. – Noob – 2015-05-07T16:07:10.600
Layer 3 packets are inside layer 2 frames as data. The bridge has not the abilities to look inside the frame to decode the data. It will look only the header of the frame. That's why IP packets with different subnets can go through the bridge, however if a PC sees that in his LAN is a packet of a different subnet, he will ignore it. – jcbermu – 2015-05-08T07:34:32.377
not sure if you are still here, if host A send a l3 packet IN subnet A through a bridge to another host B in subnet B, B will accept it, isn't it ? But from what you mentioned, the PC will ignore it. How does B know that the packet is originating from the same LAN/L2 device. – Noob – 2015-06-23T04:57:14.370
@Noob Host A in subnet A needs to have configured a default gateway in order to send packets to another subnet, otherwise host A won't create and send the packet. The default gateway, usually a router or Layer 3 device is the path between different subnets. – jcbermu – 2015-06-23T09:24:22.827
@Noob Imagine two rooms with a lot of people, you can write and pass notes to any other on the same room as you, but if you want to send a note to someone on the other room you have to give it to a messenger. Obviously you need to know who is the messenger. This messenger is the only one who can go from one room to the other and his task is to move back and forth messages between rooms. Each room is a LAN with its subnet and the messenger is the default gateway. – jcbermu – 2015-06-23T09:36:41.163
if 2 person are in the same room, but they belong to different subnet. they can still talk to one another ? – Noob – 2015-06-23T11:39:09.120
@Noob No. subnet is like the membership to the room. If your original room is A, your membership is A. So if you go to room B you can't pass notes to people in the room cause your membership is A and you are on room B. The easy solution is to change your membership (change subnet). – jcbermu – 2015-06-23T11:45:09.217