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Is a router required to set up a network?

Suppose I have just an Ethernet wire and then I fix it up to 3 computers.

Can I then say that these computers are now in a network? If they are in a network now without that router, with just the Ethernet wire alone, how and where is the network IP address and all that subnet mask going to be generated from? It is likely going to be a private address since it is local but who would assign the 3 computers with their own host IP without the router?

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closed as not a real question by Jim B, Iain, Tom O'Connor, Ward, Scott Pack Nov 10 '11 at 14:35

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5 Answers 5

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A DHCP server can still assign IPs without a router. DHCP is a layer 2 protocol and doesn't need a router to function. In fact, routers need to be configured specially to hand out IP addresses across a routed network.

The three computers would be considered on the same Layer 2 network, but their traffic wouldn't be able to leave it without a router.

If you don't have a DHCP server on the network, then the computers will autodiscover an available address on the 169.254.0.0/16 scope that is defined in RFC 3927 for just this situation.

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Thanks! Their traffic "wouldn't be able to leave it without a router" does it mean that the packets sent from the 3 computers will not leave the network? But having a router not connected to another network or internet will still have the same effect, right? So without a router, will there still be a network IP address(say the first IP address) since the computers can communicate directly without going through a router? If there is still a network IP address, is the Ethernet wire going to just somehow generate one by itself? –  xenon Nov 9 '11 at 15:47
    
@xEnOn You're all over the place, so I have no idea what you're trying to get at, but let me take a stab at it anyway. The three computers would be able to communicate with each other. Their traffic would not be able to leave that local network (to the Internet, for example, or to another subnet), but the three computers will be able to communicate with each other just fine. They will all receive IPs either statically (set by you), from a DHCP server (set up by you) of from autoconfigure as described in the RFC that I linked to. –  MDMarra Nov 9 '11 at 15:53
    
Thanks! I get what you mean now. Just one more thing, so this would mean that as I set the computers with their own IP statically or from the DHCP server, there will be no network IP address, right? Or would the 169.254.0.0/16 automatically become the network IP address and then the first usable IP address that the DHCP server or I could set is 169.254.0.1/16? –  xenon Nov 9 '11 at 15:59
    
@xEnOn The first usable address will be 169.254.0.1. 169.254.0.0 is still designated as the network address and 169.254.255.255 is designated as the broadcast address. If you are assigning addresses statically or from a DHCP server, it should not be in the 169.254/16 scope. I suggest that you actually read the RFC that I linked to. It has the answers to all of your questions. They're the standards, don't take their availability for granted. –  MDMarra Nov 9 '11 at 16:01

Is a router required to set up a network?

No. But you won't be able to just fix an Ethernet cable to three hosts -- at least I've not seen 3-way-crossover cables. You'll need, at minimum, a hub.

Can I then say that these computers are now in a network?

Yes, you can say "these computers are now in a network". They are in a network in the described configuration.

how and where is the network IP address going to be generated from?

You would need to manually assign each node an IP, or run a DHCP service on the network to assign IPs.

who would assign the 3 computers with their own host IP without the router?

You assign the IPs, either manually or with DHCP. Note that without a router, all traffic will be local to the network.

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So same for the subnet mask, without a router, I can manually set the subnet mask or let the DHCP service to decide on the subnet mask on the network? Also, I will have to run the DHCP service on all the computers, right? Thanks! –  xenon Nov 9 '11 at 15:41
    
Yes, you either manually configure the netmask, or let DHCP do it. You need only a single DHCP server, but each computer should have its interfaces configured to use DHCP config -- On Windows hosts, the "DHCP Client" must be running. –  jscott Nov 9 '11 at 15:46
    
So once the DHCP server is set up, the server computer will have its own host IP address and the other 2 DHCP client computers will have its own host IP address assigned by the server computer. Is this right? Will there still be a network address since the computers are going to send packets directly without going through a router? –  xenon Nov 9 '11 at 15:52
3  
:-) Forget about thicknet and thinnet, did you? Not that he probably has the right NIC or adapter, but you don't need a switch or even a hub for ethernet. –  mfinni Nov 9 '11 at 15:52
    
@mfinni Yes, I forgot, until you reminded me -- thanks. :) Only ever touched that stuff in the classroom. –  jscott Nov 9 '11 at 15:58

Is a router required to set up a network?

No. Only to route traffic between networks.

Suppose I have just an Ethernet wire and then I fix it up to 3 computers.

That's pretty hard to do with modern Ethernet cabling since only star configurations are supported.

Can I then say that these computers are now in a network?

Yes.

If they are in a network now without that router, with just the Ethernet wire alone, how and where is the network IP address and all that subnet mask going to be generated from? It is likely going to be a private address since it is local but who would assign the 3 computers with their own host IP without the router?

First, IP is not a network requirement. You can have a network without any IP at all.

Second, many nodes can assign themselves IP addresses in a mechanism designed specifically to make a bunch of devices connected by a switch "just work".

Lastly, human beings can assign IP addresses and subnet masks to each device. Alternatively, you can run a DHCP server on one of the machines. You don't need a router to assign IP addresses.

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ipv6 auto assigns a link local address and APIPA is a protocol designed to do the same for IPv41. A router is only required on a network when routing (getting packets to another network) is required. Ethernet cables are point to poin connections and cannot be created in a 3 headed configuration. A hub is the minimum required network device to connect more than 2 computers at one time.

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A wire connects one piece of equipment to another. You cannot connect three PCs to each other with one wire.

If you do some trickery to physically, electrically connect three wires together, what you have essentially created is a passive hub - so yes, you can use this to network three PCs.

If you are using IP on a network with no DHCP server (this is a separate question - you can have a DHCP server with a hub, or lack one with a router), the computers will each automatically grab an IP in the 169.254.0.0/16 block (or fe80::/10 in IPv6). Windows refers to this as Automatic Private IP Addressing.

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