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Ok. So I am reading this book about networking and I can't seem to have a clear picture of what that emphasized phrase mean in this paragraph, especially the word support:

"An important feature of routers is that you can use them to connect networks
that use different layer 2 protocols. For example, a router can be used
to send a packet from an Ethernet to a Token Ring network. **As long as both
networks support the same layer 3 protocol**, it doesn’t matter whether their
layer 1 and layer 2 protocols are different."

Ok. So does that mean networks "using" the same layer 3 protocol only OR networks that recognizes the layer 3 protocol?

The layers refer to the layers of the OSI model.

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1 Answer 1

up vote 3 down vote accepted

When you start looking at the OSI model, things can become very complicated.

Different level 2 protocols exist because at the time, they were better for different things (still true to some extent).

Level 1 "devices" exist as they are needed for different environments, for example IEEE 802.3 represents Ethernet and IEEE 802.11 represents WIFI, both of which are layer 1 physical layer.

Taking this a bit further, 802.11 and 802.3 whilst not compatible with each other on the physical layer, they are cross compatible further on in the model and all they require is some sort of device to make them cross compatible (e.g. a level 2 switch), therefore, they work fine through a router.

Basically put, level 3 in most environments is networking as we know it - IPv4/6 addresses, ICMP features such as ping, security through IPSec and more. But, it doesn't have to be.

It is possible to have a completely different networking infrastructure environment with your own level 3 specification, this is most commonly seen in industrial controls and similar where there are very specialised needs, these devices may use a common level 1 or level 2 feature, but, may have a completely different level 3 stack meaning that routing between the devices would be impossible.

I am sorry, but, this is a very advanced topic to try to explain, I hope I have helped, but, I fear I haven't - feel free to ask follow up questions and I will do my best to answer.

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I think you answered my question. I just need you to confirm it. So do you mean that if two networks are currently using different layer 3 protocols, they can't be routed? If so, does this mean that it is the protocols that are actually being routed and not the network? –  WikiWitz Dec 27 '11 at 13:00

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