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Networking IP Question

Y

Ynog

Golden Member
I have a question regarding a situation in networking and I am curious what will happen as a result.

Say you have 2 LANs, lets call them LAN A and B.

If LAN A and B are started separately, with two machines on each.
Now in this problem lets say that the machines on LAN A have the same
IP addresses as the machines on LAN B.

What happens if you connect LAN A and B via a switch?
 
If you are on the same network such as 192.168.0.x but none of the PCs have duplicate IP addresses then they will merge and be one big happy family.

If any of the PCs have duplicate IP addresses then you will have conflicts.
 
Originally posted by: BeanDip
If you are on the same network such as 192.168.0.x but none of the PCs have duplicate IP addresses then they will merge and be one big happy family.

If any of the PCs have duplicate IP addresses then you will have conflicts.
Click to expand...

I kind of figured there would be conflicts, but what problems arise from these conflicts.
Thats what I am looking for.
 
Probably random erratic behavior, depending on which computer happens to get the packet first? You might get an IP conflict warning, depends on your OS.

If this is a real problem, have each network's DHCP server give out different ranges of IPs. This happens sometimes with people who use VPN.
 
Originally posted by: Ynog
Originally posted by: BeanDip
If you are on the same network such as 192.168.0.x but none of the PCs have duplicate IP addresses then they will merge and be one big happy family.

If any of the PCs have duplicate IP addresses then you will have conflicts.
Click to expand...

I kind of figured there would be conflicts, but what problems arise from these conflicts.
Thats what I am looking for.
Click to expand...

Actually, windows MIGHT...as in MIGHT allow the FIRST machine with a duplicate IP to stay on the switch and communicate with PCs that have different IPs. Other IPS that are the same as the first one will not be allowed on the switch. Keep in mind that this is a rare occurance.

Usually, duplicte IPs will prevent all machines with dup. ips from transfering at all....basically, the arp will return be getting the other PC's IP and your own as well....no good.
 
Originally posted by: FishTaco
Probably random erratic behavior, depending on which computer happens to get the packet first? You might get an IP conflict warning, depends on your OS.

If this is a real problem, have each network's DHCP server give out different ranges of IPs. This happens sometimes with people who use VPN.
Click to expand...

Aye...if you want dup. IPs for some odd reason, use NAT, but still, you would need two routers, one for each subnet...$$$$$
 
Originally posted by: Goosemaster
Originally posted by: Ynog
Originally posted by: BeanDip
If you are on the same network such as 192.168.0.x but none of the PCs have duplicate IP addresses then they will merge and be one big happy family.

If any of the PCs have duplicate IP addresses then you will have conflicts.
Click to expand...

I kind of figured there would be conflicts, but what problems arise from these conflicts.
Thats what I am looking for.
Click to expand...

Actually, windows MIGHT...as in MIGHT allow the FIRST machine with a duplicate IP to stay on the switch and communicate with PCs that have different IPs. Other IPS that are the same as the first one will not be allowed on the switch. Keep in mind that this is a rare occurance.

Usually, duplicte IPs will prevent all machines with dup. ips from transfering at all....basically, the arp will return be getting the other PC's IP and your own as well....no good.
Click to expand...

Sorry for the late response.

Ok so dupilicate IP will prevent all machines with dups from transfering.

But what behavior might be expected in this case.

Two machines (A1 and A2) are connected through a switch (S1) on Lan A.
Two more machines (B1 and B2) are connected through a separate switch (S2) on Lan B.

The machines (Linux) are all booted, and get IP and communicate with each other ok (A1 to A2) (B1 to B2),
because they are separate networks.

Now my question arises is if Lan A and B are then connected, where A1 and B1 both have the same IP,
What behavior will result? I realize that ARP packets will go out. But I am not sure how the switch will
effect the situation. Will A1 bump B1 on LAN B and B1 do the same on A1's LAN?
 
Originally posted by: Goosemaster
Originally posted by: Ynog
Originally posted by: HKSturboKID
How are the IP currently configured? Static or DHCP?
Click to expand...

Static
Click to expand...

They are NOT on seperate networks. A SWITCH is a network bridge, but NOT a router, unless it is a layer 3 switch which you probably aren;t using.
Click to expand...

Sorry well aware that they aren't separate networks once the switches are connected.
I was just refering to the original connections between machines.

I figured that NAT or routers was going to be teh way to go, I was just looking for the
behavior one might expect when connecting the switches.

Also, and I think I already know the answer to this question, is there anyway to talk to another machine just based on its MAC
address.
 
Also, and I think I already know the answer to this question, is there anyway to talk to another machine just based on its MAC
address.
Click to expand...

Sure, but nothing at the application level. Things like NetBEUI, IPX, etc don't care about IP addresses.
 
damn I am an idiot....I read your question wrong🙁

The machines are only using MAC addressing if only connected to a switch, so transfers WILL work if they are MAC based such as Netbui and IPX as nothinman stated.

If you type in an IP and try to communicate, the operating system will the one to give you the error because ARP requests will be coming back to it...from itself IIRC. THe switch will prolly transfer the files to both so although you will get an error, you MIGHT get the data on the other machine.

So simple file sharing will not work using Window's TCP/IP default IIRC
 
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