When troubleshooting complete lack of connectivity, it's best to proceed in a systematic way in an attempt to isolate the problem.
It's usually a good strategy to put all your equipment into a standard condition, by doing the following:
You can join at this point even if you haven't followed the above standard-conditioning, provided you recognise that the diagnostic tests below refer to a PC connected directly to a cable modem, and that, if you have a NAT router inbetween, things will be different. If your connectivity problem only occurs with the NAT router in circuit, see NAT routers: common problems.
If your PC or Mac does not work at all when connected to the cable modem, first check that your basic network settings are correctly configured, then check the IP address which has been allocated, by following the procedure in What's my IP address?.
The possibilities at this stage are an IP address looking like:
If your IP address is absent or 0.0.0.0, then the network interface is not operational. Possible causes for this are:
If your IP address is in the range 169.254.xxx.xxx, then this indicates that the DHCP client in your computer failed to contact a DHCP server on the network, and APIPA (Automatic Private IP Addressing) is enabled, and it has automatically assigned you a private IP address in the 169.254.xxx.xxx range. With such an address, you will not be able to work with the public Internet, nor even read the cable modem diagnostic pages. Possible causes for this are:
If your IP address is in the range 192.168.100.xxx, then this indicates that you have a working connection to your cable modem, and the cable modem is working, but is not in contact with the cable network. The cable modem has an internal DHCP server which has temporarily issued you with a 192.168.100.xxx number so that you can at least look at the cable modem diagnostic pages with a web browser. With this address, you will not be able to work with the Internet. Possible causes for this are:
If you are an NTL stand-alone cable modem user with an IP address in the range xxx.xxx.xxx.248 to xxx.xxx.xxx.253 then you are on the temporary limited-access pre-registration service. You must complete your registration online: see Post-installation registration. Normally this is done as part of the broadband installation from the CD-ROM, but some incomplete installations can leave you in this state.
If you are an NTL digital TV set top box (STB) user with an IP address in the range 10.xxx.xxx.xxx, then probably you have just connected to the STB a PC or router with a MAC address that has not previously been registered with this STB. You must register the new MAC address: see First connection to set top box on NTL.
If your IP address is a normal in-service address, but the connection appears dead, try using a traceroute command to one of your ISP's servers (e.g. e-mail, web, or news) to diagnose how far into the internet you can get replies from. If you get no replies at all, there is a fault somewhere between your PC and the UBR on the cable network. If you get replies with IP addresses, but none at all translated to names, then you have lost contact with your DNS servers. If you get a normal sequence of replies with both IP addresses and DNS names, then your connection is not in fact dead at basic TCP/IP level, but there might be problems with specific services.
If your problems are evident primarily in a web browser (e.g. The page cannot be displayed), then it is possible that your problem is with a web proxy (see Web Proxy issues), and not with the network connection itself.
If your problems are exclusive to e-mail, then check your anti-virus product, as many anti-virus programs interfere with communication between e-mail clients and the e-mail servers.
If the cause of the strange IP addresses has been resolved, you can cause your PC/Mac to try its DHCP request again either by restarting, or by following the procedures in Requesting a new DHCP lease manually.
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