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For some time I have observed a strange behaviour when connecting to the internet through my WLAN. Depending on the computer I use on my network, the ISP assigns me totally different external IP addresses (some begin with 81. ... some with 221. ...). I have the strange feeling I get observed. Normally I always had a static IP beginning with 81, but this changed some weeks ago.

How can my ISP identify the computer I'm using? I tried to change my computer name and my MAC address but wasn't successful.

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Is your connection point to your ISP through the same router? What do you mean by WLAN (some people mean different things)? –  schroeder Jun 10 '13 at 16:34
    
@schroeder I have an Apple AirPort Extreme and connect every computer through this WLAN-router. –  John Jun 10 '13 at 16:54
    
Are you saying that certain machines get the same IP consistently, and that IP is different from the IPs that other machines get? Is there a difference in behaviour from wired and wireless connections? Guest/non-guest wifi network? –  schroeder Jun 10 '13 at 17:01
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This sounds completely ordinary to me. Your ISP likely has several IP address blocks that they use for DHCP, and sometimes you get an IP from one block, sometimes from the other. –  Xander Jun 10 '13 at 17:03
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@schroeder When I'm connecting one of the mentioned machines that always receive an IP beginning with 221. directly via LAN/wired and not using WLAN I get an 81. IP. So I think it could be because of the WLAN router setting as mentioned below by AbsoluteƵERØ. I'll have to check if the Bridge mode is enabled. –  John Jun 11 '13 at 5:56

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Private network IPv4 ranges are:

10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

Since the IP address ranges you're receiving are from your ISP (you can look them up on a site like arin.net) you're receiving DHCP passthru addresses because your Airport Extreme is in Bridge mode (page 31).

You can better protect your local network by disabling bridge mode, since bridge mode is meant to be used in situations where you're using your own DHCP server to give out addresses and not use the one built into the router. It makes the router act like an access point instead of a router.

This will not slow down your internet connection, but you will no longer be able to connect to your devices directly from the internet without going through the router. Then you can connect to devices on your local network from within your local network, and you'll have better control over the IP addresses you're receiving since you'll be assigning DHCP addresses from the Airport Extreme itself.

They label it in the interface as "connection sharing" which unfortunately confuses people. Also they do mention that you'll get different IP addresses, but they're talking about the IP addresses on the internet (external IPs) as opposed to local LAN addresses (which you will have when you enable DHCP outside of bridged mode).

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Thank you for your great answer! Unfortunately I'm not able to upvote your answer because I don't have enough reputation yet. –  John Jun 11 '13 at 20:23

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