Duplicate IP address
 

I think that my IP address is the same as another server's, because I recived an error message which read something along the lines of : the IP (my IP) is the same as the DHCP server : (server's MAC address).

Is this worth worrying about? Does that mean that when people try to connect to that server, they may end up connecting to my Mac?

I closed the error window, is there a way that i can view it again, perhaps a log or something?, so that i can give you more information.

Did you manually set your IP? If you are using DHCP, you should leave that field blank; the DHCP server will assign an IP for you. Under no circumstances that I can think of should you have the same IP as the DHCP server. If you really do, it's going to cause problems.

Joe VanZandt

The usual reason for getting error messages about duplicate IP addresses is that someone on your network has manually assigned an IP address that is in the range of the IP addresses being assigned by a DHCP server, or that there are two DHCP servers on the same network (e.g. someone added a router that does DHCP without realizing the problems this can cause)

I think i am using DHCP, but i that i used the IP automatically assigned to me.
I am on a small PC/Mac home network with one router.

If it helps, the devices are: 3 PCs, One Mac, One Nintendo Wii, two Nintedo DS's.

You can probably see messages about this in system.log
(Look at logs using the "Console" application from /Applications/Utilities)

Look in the Network preferences to check on your DHCP settings and to see what your IP address and MAC (hardware) addresses are.
Is your Mac connected via Ethernet or via Airport?

You can also get the above info by using the command:
ifconfig
in a Terminal window.

Yes, but people turn on DHCP servers on their computers fairly often without realizing it, making it easy for more than one DHCP server to be on a network. The 'Internet Sharing' function that is included with both Windows and OS X turns the computers running it into (poor) DHCP servers. This function should not be used--it's a kludge. A real router is always a better solution. (Hardcore nerds will disagree, and say that a computer with two network cards running routing software is better than a small home router. While I won't argue with that, I would say that a computer whose sole purpose is to be a router IS a real router. But that's very different than someone who turned on 'Internet Sharing'.)

Trevor

I got the error again, so i took a screenshot, but you'll need to change the extension back to ".PICT" again.

Please launch the "Terminal" application (under /Applications/Utilities) and copy & paste the following commands, press Return, then copy & paste the results back here so we can see:

ifconfig
ipconfig getpacket en0
ipconfig getpacket en1
echo "done"

It looks like one of the PCs is using a static loal address. That Mac address is from Intel:

00-13-20 (hex) Intel Corporate
001320 (base 16) Intel Corporate
Lot 8, Jalan Hi-tech 2/3
Kulim Hi-Tech Park
Kulim Kedah 09000
MALAYSIA

Terminal says:
"Last login: Sat Jun 9 16:46:24 on ttyp1
Welcome to Darwin!
iMac:~ Tris$ ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::216:cbff:fe87:ea89%en0 prefixlen 64 scopeid 0x4
inet 192.168.0.3 netmask 0xffffff00 broadcast 192.168.0.255
ether 00:16:cb:87:ea:89
media: autoselect (100baseTX <full-duplex,flow-control>) status: active
supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP <full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX <full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT <full-duplex,hw-loopback> 1000baseT <full-duplex,flow-control> none
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:16:cb:01:29:50
media: autoselect (<unknown type>) status: inactive
supported media: autoselect
wlt1: flags=41<UP,RUNNING> mtu 1500
fw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 2030
lladdr 00:16:cb:ff:fe:47:04:0c
media: autoselect <full-duplex> status: inactive
supported media: autoselect <full-duplex>
iMac:~ Tris$ ipconfig getpacket en0
op = BOOTREPLY
htype = 1
flags = 0
hlen = 6
hops = 0
xid = 1900045731
secs = 0
ciaddr = 0.0.0.0
yiaddr = 192.168.0.3
siaddr = 0.0.0.0
giaddr = 0.0.0.0
chaddr = 0:16:cb:87:ea:89
sname =
file =
options:
Options count is 7
dhcp_message_type (uint8): ACK 0x5
server_identifier (ip): 192.168.0.1
lease_time (uint32): 0x3f480
subnet_mask (ip): 255.255.255.0
router (ip_mult): {192.168.0.1}
domain_name_server (ip_mult): {192.168.0.1}
end (none):
iMac:~ Tris$ ipconfig getpacket en1
iMac:~ Tris$ echo "done"
done"

By the way, now i am using a ethernet cable to conect to the router (the router is a 3-in-1 router, switch and modem).

The above shows that your Mac's IP address (on Ethernet) is 192.168.0.3 and the router's IP address is 192.168.0.1 (as you said).

The message you received is therefore not about your IP address - the message referred to IP address 192.168.0.2
Is it possible that you had configured your Network preferences to use a static IP address and you had chosen 192.168.0.2 ?
Why did you refer (in post #1) to this address being your Mac's IP address?

Just being curious, I wonder what would turn up if you ran IP Scanner? It scans your entire network to show everything that's connected. Once it's found everything, you can then click on any found item to see its MAC address.

I hate sites that only use flash so you can't give a direct url. Click on the IP Scanner icon and then look at the thumbnail picture to get some idea of what you can get with IP Scanner.

Personally, I have fun using it when I can get into someone else's network to figure out what stuff I can get into (non-destructively only). ;)

Versiontracker has a better description of IP Scanner.

FWIW: Another cause of "already in use" errors is braindead DHCP servers.

a) Server assigns addresses starting at {say} 10.99.88.7 ...

b) Gives out .7, 8, .9, 10..

c) Server gets powercycled as part of maint or someone trips over cord.

d) New user shows up, server starts all over and assigns .7 again.

e) Both original .7 and new one gripe, confusion reigns..


A smarter DHCP server:

1) picks address..

2) tests to be sure it's not in use...if it is, it goes past and ....

I ran IP scanner and it found (other than my Mac-the host) the family PC and the router, and neither of them had similar IPs to me. My brother's laptop was off at the time i scanned, so i will scan again later when it is on, to see if that makes any difference.

Turns out that it's the router that shares my IP. Can you change a routers IP?

You can change a router's local IP, in other words my router at home is 192.168.4.1 changed from the .0.1 that is standard. But you cannot change it's WAN address which is assigned by your ISP. In my example I get my external IP from the Cox Cable system which also uses DHCP I believe.

Changing the router's IP isn't an option (dont ask why, i don't know myself), could i change my Mac's IP instead or set it not to display that error?

No you can't stop your mac from complaining about that because the network will not work.
You should be able to tell the router to start it's range of addresses at another number say 15-30 instead.

You could try renewing the Mac's DHCP lease. That may provide you with a new address. You could also try manual addresses in the range from 192.168.0.2 to 192.168.0.252 until you find one that isn't in conflict. Your subnet mask will be 255.255.255.0, the router will be 192.168.0.1, and if you know your DNS settings it's a good idea to include them, but they aren't absolutely necessary since the router should take care of that.

How do i renew the Mac's DHCP lease? I can't change any of the router settings(wish i could but i cant).

How do i renew the Mac's DHCP lease? I can't change any of the router settings(wish i could but i cant).

Click on the Renew DHCP Lease button in Network preferences.

Thanks, cwtnospam, i think that's it fixed now.