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If I run ifconfig on my Linux (Ubuntu) server, I get the following output:

eth0      Link encap:Ethernet  HWaddr d0:27:88:0f:80:74
          inet addr:  Bcast:  Mask:
          inet6 addr: 2002:5eaa:df8d:0:fcb8:c688:1a5b:f9a0/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:f548:80e4:7e2d:d4b9/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:302c:7527:3157:172a/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:885a:ccef:87bd:4d8d/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:b9cd:1097:d233:71b7/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:c29:cfd9:9913:ea39/64 Scope:Global
          inet6 addr: 2002:5eaa:df8d:0:d227:88ff:fe0f:8074/64 Scope:Global
          inet6 addr: fe80::d227:88ff:fe0f:8074/64 Scope:Link
          inet6 addr: 2002:5eaa:df8d:0:183b:3403:3ba0:3431/64 Scope:Global
          RX packets:74163188 errors:0 dropped:0 overruns:0 frame:0
          TX packets:87047108 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3705441868 (3.7 GB)  TX bytes:2209135524 (2.2 GB)
          Interrupt:43 Base address:0x2000

lo        Link encap:Local Loopback
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:28989 errors:0 dropped:0 overruns:0 frame:0
          TX packets:28989 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:2264373 (2.2 MB)  TX bytes:2264373 (2.2 MB)

As far as I know, I'm connected on my internal network over IPv4. So, why do I have so many inet6 addr items listed here?

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up vote 6 down vote accepted

Ubuntu has IPv6 enabled by default and has for a long time. One feature IPv6 has is the ability to generate random addresses with a /128 range. This is a privacy extension as your IP address will change periodically and can't be traced back to your server as easily as a fixed address. These temporary addresses are kept active for quite a while after they have been replaced.

It appears you have a radvd (route advertisement daemon) running on your network announcing 2002:5eaa:df8d:0::/64 as a base for generating addresses. Your server is generating temporary addresses in this subnet. The address beginning fe80: is a local address and should be stable.

ifconfig doesn't give much information about IPv6. The ip command will give you more details on your addresses. Try the command ip -6 addr to get information about the lifetimes of the various addresses.

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Fascinating. Interesting to know that the ip command might be more useful as a Linux admin/developer going forward into the IPv6 world. Thanks for the in-depth response. – gpmcadam Oct 20 '12 at 1:24

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