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Layer 2 (Ethernet layer) recognizes the MAC address using ARP protocol. It sends a question to the whole network asking: "Which MAC address has the IP x?". That's how it recognizes the destination MAC address.

But how does Layer 3 (IP layer) knows the IP address? It is not passed to it from the transport layer (layer 4). Only the port number is passed.

I hope I am specific. Thanks in advance.

Matt
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Tony
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    'Only the port number is passed': where did you get that from? Only the port number *isn't*passed. The network layer doesn't know about port numbers. Only the transport layer knows that. But the network layer certainly knows about IP addresses. That's what it's for. – user207421 May 01 '18 at 03:20

3 Answers3

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There are two IP addresses associated with a packet, the sending (also know as source) IP and destination IP.

Sending IP - The sending IP is trivial, it is a configuration parameter of the network interface by which the packet will be transmitted. It is configured either statically or more typically in modern networks through DHCP.

Destination IP - The destination IP can be slightly more complex depending upon whether the destination computer is local or remote to the sending computer. In this case, remote means that the destination computer is not located in the same subnet as the source computer.

For remote destinations DNS servers are used for name<->IP resolution. DNS is what allows humans to use our strength, names such as www.espn.com, while computer can use their strength, bit representation of IP addresses such as 10.5.0.1. DNS resolution can be configured many different ways, but basically boils down to examining local cache to see if a prior resolution exists, and if not querying one or more DNS servers to resolve the request.

If the destination machine is local, on the same subnet, then ARP/RARP is used for MAC<->IP translation. Once again a cache is checked first, in this case the ARP cache, before creating a new ARP request (and thereby using network bandwidth).

I hope this helps.

Gerald P. Wright
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    Thank you for your response. But, my actual question is: What Layer 3 receives from Layer 4, does NOT contain anything about the IP address (or host name). So, from where does it brings it? By other words, is there a common or global source of information available simultaneously to the 7 layers, or the case is that every layer can only get information from the one above it? I hope I am clear – Tony Jan 10 '12 at 08:35
  • @Tony You are mistaken in your fundamental assertion. – user207421 Oct 20 '14 at 09:18
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Every layer provides an interface to the layer above. It is through this interface that layer4 sends layer3, the destination IP address. layer5 (Application Layer) sends the destination IP address it wants to talk to, to layer4 which in turn sends it to layer3. Layer3 includes this in the IP packet and passes this packet to the layer2 (along with other things like MAC address of the destination etc..)

It is like giving arguments when you call a function. Suppose the interface provided by layer3 is send_packet(), layer4 calls it, may be, like send_packet(srcIP, destIP)

nitishch
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But how does Layer 3 (IP layer) knows the IP address? It is not passed to it from the transport layer (layer 4). Only the port number is passed.

This isn't correct. Both TCP or UDP have an explicit bind() operation which does indeed pass an IP address. TCP also has an explicit connect() operation which also passes an IP address, and an implicit bind() operation where the source IP address is computed from the unicast IP routing tables.

user207421
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    And the port number *isn't* passed. Port numbers only exist at the transport layer. – user207421 May 01 '18 at 03:17
  • Sorry to ask but I'm new to the TCP/IP protocol stack and this question has also been bugging me lately. If TCP and UDP pass the IP address, doesn't it violate the layering philosophy? If TCP/UDP are part of the transport layer (layer 4) and IP is layer 3, then TCP/UDP would need to have the layer 3 info for the `bind()` operation? Or is this one of the cases where in practice there are exceptions to the ideal layered model? – Andrei Poehlmann May 26 '21 at 11:19