Cable Modem Troubleshooting Tips: IP addresses

An IP (Internet Protocol) address consists of four decimal numbers in the range 0-255 separated by dots, such as 62.253.135.64. Every computer on the internet needs a unique IP address. For many of the procedures described elsewhere on this site it is necessary for the user to discover and use various IP addresses: the instructions for finding the following IP addresses are listed on this page:

What's my IP address?

When you connect to the cable network, your PC/Mac is automatically issued with a unique IP address by means of DHCP (Dynamic Host Configuration Protocol). The public IP address allocated by DHCP for the cable connection is called the WAN address to distinguish it from any private LAN addresses you might be using if you have a home LAN. If your PC/Mac is connected directly to the cable modem, with no intervening routers, then its IP address will be the WAN address.

What's my WAN IP address?

If your PC/Mac is part of a home LAN behind a NAT router, then the above procedures will tell you the private LAN address of the PC/Mac, not the public WAN address allocated to your cable connection. To discover your public WAN address, visit one of these sites:

Your NAT router might also have configuration pages or programs which will tell you the public WAN address. For instance, for the Linksys routers, send your web browser to http://192.168.1.1/ and click the tab Status:

On the NTL cable modem service, the possible in-service IP address ranges are:

62.252.xxx.xxx to 62.255.xxx.xxx.
80.0.xxx.xxx to 80.7.xxx.xxx.
81.96.xxx.xxx to 81.111.xxx.xxx.
82.0.xxx.xxx to 82.31.xxx.xxx.
213.104.xxx.xxx to 213.107.xxx.xxx.

It is quite normal for one UBR to have more than one IP address range assigned for customers, and these ranges might be quite different in their leading digits. When you are allocated a fresh IP address by DHCP, it is as likely to come from one range as another: a change of IP address range is not a cause for concern.

Finding the Default Gateway address

The Default Gateway (or Router) is your link to the internet: your PC will send IP packets to, and receive IP packets from, the Default Gateway (except when the packet is to or from another IP address on the same sub-net as yourself, in which case it goes direct). On a cable network, the Default Gateway address is physically part of the UBR (head-end).

It is very common for the Default Gateway address to end in .1 or .254, with the first three parts being the same as the public WAN IP address for the connection.

If your PC/Mac is connected directly to the cable modem, you can discover the Default Gateway address of your connection as follows:

If your PC is on a home LAN connected via a NAT router to the cable modem, use the configuration pages or program for the NAT router to discover the Gateway address. For instance, for the Linksys routers, send your web browser to http://192.168.1.1/, click the tab Status, and look for the line Default Gateway. See example above.

Finding the cable modem's address

The customer-side IP address of a DOCSIS cable modem is normally 192.168.100.1. For 3Com Tailfins with early firmware versions it was 149.112.50.65. Firmware can be automatically updated by your cable operator down the cable, so by the time you read this, most Tailfins will have been updated to use the standard address. The 3Com CMX does not have a customer-side IP address.

The IP address 192.168.100.1 will be present even if no web diagnostics are offered on that address.

The cable modem IP address 192.168.100.1 is not in the same sub-net as the user's PC. So, when trying to send to 192.168.100.1, the user PC's IP stack will normally route the packet to the Default Gateway address at the UBR. Since no routes exist to the private address 192.168.100.1 (and there are multiple instances of this IP address on any one CATV segment), the UBR drops the packet. This would mean that in theory the PC could never talk to the cable modem. However, the Surfboard, the 3Com Tailfin, and the ntl:home 100/120 are capable of sniffing the passing traffic through the transparent bridge to intercept any packets addressed to themselves. This only works when the bridge is open, so the cable modem diagnostics cannot be read when the cable modem is booting up or failing to remain in contact with the UBR.

For cable modems other than the Surfboard, Tailfin and ntl:home 100/120, and for the Surfboard/Tailfin when their bridge is closed, the only way to communicate with them is:

Note that, with the Surfboard and ntl:home 100/120, when the cable modem is not online to the cable operator's network, it runs an internal DHCP server which automatically gives the PC an IP configuration on the 192.168.100.xx sub-net to enable the PC to communicate with the cable modem on 192.168.100.1.

Finding the UBR address

The UBR in fact has more than one IP address. One of the public IP addresses that it has is the WAN Default Gateway address that is allocated to your PC by your cable ISP. You should use this Default Gateway address when you wish to request a service (e.g. time synchronisation) from the UBR.

The other address for the UBR will usually be a private IP address, which will be the apparent source of IP packets sent from the UBR itself to your PC. You should use the private IP address for the UBR, for instance, when configuring firewalls to permit the UBR to send to your PC, or when using the -ubrip option of the DocsDiag program. To find the private IP address of the interface of the UBR to which your cable modem is connected, perform a traceroute to anywhere. The private IP address of the UBR will normally be returned in hop 1 of the traceroute results:

If you have a home LAN with a NAT router, then in some cases, the NAT router inserts itself as a new first hop, in which case the IP address of the UBR will be returned in hop 2:

The Linksys NAT routers are an example of a router which does not insert an extra hop. The SMC Barricades are an example of a router which does insert an extra hop. The documentation or setup configuration of the home LAN router will normally tell you which IP address range it uses, so that it can be recognised in hop 1 above. Any 192.168.xxx.xxx address in hop 1 is more likely to be a home router than the UBR. But some home routers (e.g. Apple Airport) use the 10.xxx.xxx.xxx IP range, which can be confused with a UBR in the same range. When in doubt, disconnect the router, connect the PC directly to the cable modem, and try the traceroute again.

For original NTL franchises, the UBR will have a private IP address of the form 172.xx.xxx.254 or 10.xxx.xxx.254; for ex-C&W digital set-top-boxes, it will be of the form 10.xxx.xxx.1. For Blueyonder, the UBR will have an address of the form 10.xxx.xxx.1.

Why the UBR address is not always the same as the Default Gateway

Every IP sub-net needs a Default Gateway defined. All IP packets destined for hosts off the sub-net will be sent to the gateway for routing onwards. If your IP address is aaa.bbb.ccc.ddd, then the sub-net usually consists of all IP addresses of the form aaa.bbb.ccc.xxx, where xxx is any number in the range 1 to 254, and the gateway address could be aaa.bbb.ccc.1 or aaa.bbb.ccc.254. On a cable system, the gateway will physically be the UBR itself. On a normal ethernet, one would expect that packets from the gateway to you would show their source to be the gateway address. But on a cable system, this is not always the case. A single cable distribution system can host more than 253 customers, and so requires more than one customer sub-net active on the cable. A single UBR interface therefore functions as a gateway for more than one customer sub-net, each of which might be in different IP address ranges. So this single UBR interface is labelled with more than one IP address. Each of the customer sub-net public gateway IP addresses are treated as secondary IP addresses for the UBR interface, which is given a primary IP address in the private range of 10.xxx.xxx.1 or 172.xx.xxx.254. [This also enables the UBR to serve as the sub-net gateway for the CATV side of all the cable modems, which also have private 10.xxx.xxx.xxx or 172.xx.xxx.xxx addresses]. The upshot is that when the UBR originates a packet to an end-user (such as responding to a traceroute probe), the packet shows a source address of 10.xxx.xxx.1 or 172.xx.xxx.254, which does not appear to be an address on the end-user's sub-net, although physically it is. This private UBR address will have the same MAC address as the customer sub-net Default Gateway addresses have, because all these addresses are on the same physical UBR interface.

If you traceroute through the gateway, the UBR's private IP address will reply.

Finding the DNS server address(es)

Finding the DHCP server address(es)

You can discover the address of the DHCP server from which your PC/Mac obtains IP address leases by:

If the PC/Mac is on a home LAN behind a NAT router, then the DHCP server address obtained above might be the LAN address of the NAT router itself, providing a local DHCP service. If you want to discover the WAN DHCP server address of your ISP when your PC/Mac is on a home LAN:

With NTL stand-alone cable modems, there was originally just one DHCP server per region, but there will be a steady conversion to having two DHCP servers per region, starting in 2002. Each will act as a backup to the other. If you are writing firewall rules, you need to trust both DHCP servers. You can find the address of one of them by the procedures above. To find the address of the other:

Take the known DHCP server IP address as found above, and look up its DNS name: the result should look something like dhcpN-XXXX.server.ntli.net where:

XXXX is a region code of 3 or 4 letters;
N is either 1 or 2.

In that DNS name, alter the N field to its other possible value, then look up the IP address for that name. This will yield the IP address of the other DHCP server. If the lookup fails, there isn't a second DHCP server for your region.

With NTL digital TV STBs, there are two DHCP servers per region, each acting as a backup to the other. Their IP addresses will be of the form 10.0.xxx.70 and 10.0.xxx.71. The methods above will tell you the active one for your PC, enabling you to discover the value of xxx. If you are writing firewall rules, you need to also add the other one of the pair.

Blueyonder users have two DHCP servers, each acting as a backup to the other. They might be the same as your assigned DNS server addresses, but can be different. If you are writing firewall rules, you need to trust both DHCP servers. To determine the other address:

take the known DHCP server IP address as found above, and look up its DNS name: the result should look something like pXXsNNNo.blueyonder.co.uk where:

XX is one of the two-letter codes listed in Decoding your DNS name.
NNN is either 001 or 002.

In that DNS name, alter the NNN field to its other possible value, then look up the IP address for that name. This will yield the IP address of the other DHCP server.

IP Address:	213.107.117.246
Subnet Mask:	255.255.255.0
Default Gateway:	213.107.117.254
DNS:	194.168.4.100 194.168.8.100 0.0.0.0
DHCP Remaining Time:	2:47:44

Cable Modem Troubleshooting Tips

IP addresses