Why is tracert showing a private IP address right after my router, even though it has a public IP?

Solution 1:

The line 89-75-22-81.infra.chello.pl at the top of the traceroute suggests you are using a cable connection. Chello is a brand formerly used by UPC, a cable internet service provider. The appearance of an IP address in a private range immediately after your local network is normal for cable connections.

The address 10.216.128.1 belongs to a cable modem termination system (CMTS). It is sometimes referred to as a Universal Broadband Router (uBR), though I believe that is exclusively a Cisco term. Its function is roughly equivalent to that of your cable modem. Only part of your internet connection runs via the coaxial cable between you and your ISP. At home, your cable modem translates between coax interfaces on one side and ethernet interfaces on the other. In the same way, your provider hooks up the coax cables to the rest of their infrastructure via a CMTS. The main difference between the two pieces of equipment is that a single CMTS often serves thousands of cable modems. Even the tiny Cisco uBR7100 below can handle up to 2000 clients.

The subscriber side of a CMTS is basically a dead end in the infrastructure and does not need to be available to anyone but the subscribers. It is therefore very practical for it to have an IP address in a private range, which is what you're seeing by executing a traceroute from your machine. This again is equivalent to your modem/router at home, which will have both a private and a public address. The private one appears on your traceroute: 192.168.1.1.

A trace to your address does not show 192.168.1.1 at the end, even though in both cases it is the same device responding.

15    40 ms    39 ms    39 ms  84.116.192.101
16    37 ms    37 ms    39 ms  89-75-22-82.infra.chello.pl [89.75.22.82]
17    45 ms    48 ms    45 ms  89-66-132-177.dynamic.chello.pl [89.66.132.177]

Trace complete.

Based on these traces, I've drafted the diagram below to visualise the network.

Click to enlarge.

For the purpose of explaining the nuts and bolts of your connection to the internet, it is unfortunate that the CMTS does not decrease the packet's time to live in both directions (when the TTL runs out, hosts return an error message to the source, which is how traceroute compiles its list). This is not uncommon; like ordinary network switches, CMTSs operate on layer 2 of the OSI model, but not all CMTSs are configured this way. For example, a trace to me would list the following as the last hop before the destination (note the descriptive 'ubr' in the hostname):

213.51.138.75    emn-rc0001-ubr014-te3-0-0-202.core.as9143.net

Network-tools.com has a useful tool for executing a trace to yourself from elsewhere on the internet.

I've never actually worked with these kinds of systems, so my understanding of the subject is quite limited. Nevertheless, I hope I've been able to shed some light on why a private range IP address appears in your traces and what its purpose is.