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From Tanebaum's Computer Network:

A five-layer network is illustrated in Fig. 1-13. The entities comprising the corresponding layers on different machines are called peers. The peers may be software processes, hardware devices, or even human beings. In other words, it is the peers that communicate by using the protocol to talk to each other.

I don't understand what a peer is, because the usage of English word "comprising" can be ambiguous ( A whole comprises parts. Parts comprise a whole.)

Can you point/circle out what part in Figure 1-13 is a peer?

Are a peer and a host in the figure the same concept?

Thanks.

enter image description here

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  • "...it is the peers that communicate by using the protocol to talk to each other" – Robert Harvey Feb 23 '15 at 6:24
  • is a peer the same as a host? Or does a peer exists inside a layer of a host? Or ...? – Tim Feb 23 '15 at 6:30
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    As a non-native English speaker, for a more generic understanding of the word peer, thing of a peer as "someone or something that has the same role/rank/status as the object/person in question". So if you're a student, your fellow-students are your peers. Your tutor is not a peer to you. Your tutor does have peers, that would be the other tutors in that school. Another example, if you have job, your colleagues are your peers, but your superior is not. This can be extended to objects/nodes/layers in computing. – funkwurm Feb 23 '15 at 9:15
  • Think of a peer as an entity participating in a service, usually in the context of being a receiving endpoint but not always. – Thomas Carlisle Apr 15 '17 at 12:39
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In figure 1-13, there are many peers.

  • Host 1, Layer 5 is a peer to Host 2, Layer 5.
  • Host 1, Layer 4 is a peer to Host 2, Layer 4.
  • Host 1, Layer 3 is a peer to Host 2, Layer 3.
  • Host 1, Layer 2 is a peer to Host 2, Layer 2.
  • Host 1, Layer 1 is a peer to Host 2, Layer 1.

Note that it is commutative, so for example,

  • Host 2, Layer 5 is a peer to Host 1, Layer 5.

But to make sure you don't misunderstand, a peer is not always a layer. If I talk to you on a telephone, you and I are peers in that communication.

In the same telephone call, let's say they are mobile phones. The mobile phones each convert the audio to/from radio waves to a tower. In this case, the mobile phones are peers on the phone network.

However, in the same example, I am not a peer with your mobile phone because we are on different levels.

I hope this clears it up.

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A peer is something or someone that is an equal member of a group, where there's some kind of relationship between the members of the group. For example, "a jury of your peers" means people with the same (legal) status as you.

For networking and distributed systems; a lot of it is either "client server" (HTTP, FTP, NTFS) or "hierarchical" (NTP, NNTP, DNS); where a client and server are not peers because they aren't equal, and where clients communicating with the same server aren't necessarily peers because there may not be a relationship between the clients.

For the diagram you've posted; from one perspective it's impossible to know what the protocols are and therefore not possible to say if the hosts are peers or not (e.g. Host1 could be DHCP server and Host2 could be DHCP client, and in that case they wouldn't be equal and couldn't be considered peers).

However, from a different perspective (at the physical layer) it's extremely likely that the hosts are considered equals (regardless of which higher level protocols are being used) and from that perspective the hosts would be considered peers.

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actually peer is an end user in a network, whether it may be a phone network or an Internet network, the end device to which service is delivered is called a peer.

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