Basic Internet Architecture For the curious reader: Under
Basic Internet Architecture For the curious reader: Under many versions of Unix (for example, FreeBSD or Linux) the command ifconfig shows a variety of details about a host s currently attached link layers, including the currently assigned IP address(es), subnet mask(s), and MTU(s). If you are running Windows XP or something similar, the command ipconfig /all at a console window will also print out a variety of details about the host s current configuration. 4.2.3.5 First and Last Hops Forwarding tables are not just for routers. Hosts also have a limited forwarding table that tells them the initial next hop (often referred to as the first hop) for outbound packets. The first hop will either be to a router (for traffic destined beyond the local subnet) or directly to a neighbour on the same subnet. In cases where the next hop goes directly to a neighbour on the same subnet it is referred to as the last hop. (The first and last hops may be one and the same in the case of communication between two hosts on the same subnet.) Forwarding tables have special rules for subnets that are directly attached to one of the host s or router s IP interfaces. Rather than having a specific entry for every IP interface reachable on a local subnet, the next-hop IP address is copied directly from the packet s destination address field. The ARP cache is then scanned for a match to this next-hop IP address, and the packet transmitted to the link layer destination found in the ARP cache. Hosts typically only have a few entries in their forwarding table, for example, one entry for the directly attached subnet, and a default route (network/prefix 0.0.0.0/0) pointing to a router on the local subnet that provides access to the rest of the network. If a host has link interfaces to multiple IP subnets, it will have forwarding table entries for each directly attached subnet, and possibly multiple forwarding entries for nonlocal traffic. For the curious reader: If you are running Windows XP or something similar, entering route print in a console window will show the current forwarding rules. Under many versions of Unix (for example, FreeBSD or Linux) the command netstat -rn will show the host s current forwarding rules. 4.2.3.6 Tunnels as Links Links are simply mechanisms for getting an IP packet from one router to another. A link may even be an IP network in its own right. Transmission of IP packets within other IP packets is known as IP tunnelling, and the link is known as an IP tunnel.From the perspective of the outer IP packet, the packet being tunnelled is just another payload (as uninteresting as a TCP or a UDP frame). From the perspective of the tunnelled packet, the tunnel looks like another link layer. From an implementation perspective, an IP tunnel is a link layer where source and destination addresses also happen to be IP addresses. The tunnel s endpoint is the IP interface identified by the destination IP address in the outer packet s header. When the outer IP packet reaches its destination, the original (inner) IP packet is extracted and processed as though it had arrived over a regular interface. The outer packet s IP Protocol Type identifies the payload as a tunnelled packet, for example, protocol type 4 indicates that the payload is an IPv4 packet [RFC2003]. Because a tunnel represents a single hop from the perspective of the tunnelled packet, its TTL is decremented by one (rather than the number of hops between the tunnel endpoints).
Note: If you are looking for good and high quality web space to host and run your application check Lunarwebhost Inexpensive Web Hosting services