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From what I've read, 10.0.0.0 - 10.255.255.255 is a private address space. So I ran a tracert over the internet and I saw an address 10.15.25.98. How is that possible? Thanks in advance!

Tracert Log:

1   192.168.1.1 1ms None    
2   ----    Timeout n/a 
3   10.15.25.110    13ms    None    
4   10.15.8.49      16ms    None    
5   65.46.186.97    11ms    ip65-46-186-97.z186 
6   216.156.0.185   24ms    vb1730.rar3.chicago 
7   ----    Timeout n/a 
8   216.1.123.54    48ms    None    
9   66.109.6.154    51ms    ae-1-0.cr0.chi30.tb 
10  107.14.19.61    62ms    None    
11  65.29.1.35      58ms    be1.clmkohpe02r.mid
Alex McKenzie
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pkSML
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  • Please share the details of your tracert, you can anonymize where necessary. – MikeDawg Feb 04 '14 at 22:35
  • It's in CSV format.[code]1,192.168.1.1,1ms,None,---- 2,----,Timeout,n/a,---- 3,10.15.25.110,13ms,None,---- 4,10.15.8.49,16ms,None,---- 5,65.46.186.97,11ms,ip65-46-186-97.z186,---- 6,216.156.0.185,24ms,vb1730.rar3.chicago,---- 7,----,Timeout,n/a,---- 8,216.1.123.54,48ms,None,---- 9,66.109.6.154,51ms,ae-1-0.cr0.chi30.tb,---- 10,107.14.19.61,62ms,None,---- 11,65.29.1.35,58ms,be1.clmkohpe02r.mid,----[/code] ... Sorry, can't get this to display right... (I'm a new user) – pkSML Feb 04 '14 at 22:37

3 Answers3

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Not every ip address on your tracert has to be a public IP.

It might be a router or server on one of the internal nodes of an ISP that you went through on the way to your destination.

For a more detailed overview of how tracert works, take a look at this Computerphile video.

Alex McKenzie
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  • Just beat me to it! Good, simple and quick answer :) +1 – xstnc Feb 04 '14 at 22:40
  • Please explain to me how traceroute can determine that private IP address is in the path. I thought each host in the path basically has to be pingable... which it is in when I ping the particular 10 address in the path I'm currently looking at. But won't I then break connectivity if I use that particular address -- or subnet of addresses -- locally? Which is basically against the rules of private IPs? – Colin 't Hart Apr 18 '23 at 14:39
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    @Colin'tHart The addresses don't need to be pingable. traceroute has a somewhat hacky way of working. Your computer will attempt to send a UDP packet with a very small TTL to the destination. When a gateway along the route gets a packet with an expired TTL, it is supposed to send back a "TIME_EXCEEDED" to the original sender of the packet with its IP address, which can be internal. This is why traceroute will sometimes not return IP addresses some of the hops along the way. Those gateways are probably discarding the packet instead of sending a response. – Alex McKenzie Apr 29 '23 at 03:06
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Your ISP is likely using Carrier Grade NAT.

You can verify this by looking in your home router and checking what the WAN IP address is that you get from your ISP. You've likely been assigned an IP in the 10.0.0.0/8 range as well. If that is the case then those suspicious hops are just the routers that are used to get from your machine to the edge of your ISPs network.

heavyd
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  • The hops can still be the routers even if it is **not** the case that the external IP address of the customer-premises equipment is in a non-publicly-routable range. – JdeBP Feb 05 '14 at 10:51
  • My public IP address is a routable address. But your link gives me something more to learn ;) – pkSML Feb 05 '14 at 12:14
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If you refer to RFC1918 http://www.faqs.org/rfcs/rfc1918.html it states:

3. Private Address Space

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private internets:

     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

10.0.0.0/8 is a private IP address.

MikeDawg
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    ... which the questioner said in the question. The actual question, which you've not answered at all, was _how_. – JdeBP Feb 05 '14 at 10:43