Some cable users run a VPN client to connect to a workplace VPN.
If the cable ISP runs a transparent web proxy (as do NTL and Blueyonder), this is likely to prevent a VPN client being able to connect to port 80 of a remote VPN server. The remedy is for the VPN server administrator to accept VPN client connections on a port other than 80.
If your PC is connected via a Network Address Translation (NAT) Router (either your own, or one operated by the cable ISP), VPN connection attempts carried by IPSec may fail because the client IP address seen by the server as the source of the packets does not match the source IP address encrypted within the packets. The remedy is for the VPN server administrator to disable the checking of the source IP address.
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