The best way to find problems with your Wide area network connections is to use the traceroute utility provided with most operating systems (the notable exception being Sun Solaris X86). Traceroute can tell you the path your packets travel as they leave your system and head for their destination. Traceroute can tell you how many routers your packets travel through, how long it takes to travel between routers, and, if the routers have DNS entries, the names of the routers and their network affiliation and geographic location.
Traceroute works by exploiting a feature of the Internet Protocol called TTL, or Time To Live. Originally, this data field in an IP packet header was supposed to contain a value representing the actual amount of time a packet could be flying around the Internet before a router would simply discard it. This was designed to eliminate packets which were endlessly looping around the network and therefore wasting bandwidth and router capability. However, for a variety of reasons, the idea of a fixed amount of time was discarded. The TTL field is now interpreted to indicate the maximum number of routers a packet may transit. Each router that handles a packet will decrement the TTL count field in the ICMP header by 1. When the count reaches zero, the packet will be discarded and an error message will be transmitted to the originator of the packet.
Traceroute sends out a packet destined for the destination you specify. It sets the TTL field in the packet to 1. The first router in the path receives the packet, decrements the TTL value by 1, and if the resulting TTL value is 0, it discards the packet and sends a message back to the originating host to inform it that the packet has been discarded. Traceoute records the IP address and DNS name (if available) of that router, then sends out another packet with a TTL value of 2. This packet makes it through the first router, then times-out at the next router in the path. This second router also sends an error message back to the originating host. Traceroute continues to do this, recording the IP address and name of each router until a packet finally reaches the target host, or until it decides that the host is unreachable. In the process, traceroute records the time it took for each packet to travel round trip to each router. Slow links are therefore pinpointed.
If you cannot reach a particular site on the Internet you should run a traceroute to the site before calling your ISP to inform him that his network MUST be down. Most often you will discover that your ISP's network is just fine, but the network your favorite site is on is actually the one that is down or very slow.
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