Comments on: Do routers change packet IP info at each hop?

By: Article Write

Article Write — Thu, 10 Mar 2011 22:01:59 +0000

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By: gregbme

gregbme — Tue, 28 Sep 2010 13:58:35 +0000

Ignore my 2:14 response. It was

wrong and other postings were correct. During a normal pass through a router the IP addresses in the datagram DO NOT change. Some header information (e.g. TTL and checksum) changes and fragmenting may occur, but the IP addresses stay the same.

By: gregbme

gregbme — Tue, 28 Sep 2010 02:14:03 +0000

Professor is right, if you understand what she is saying. Remember the protocol stack and encapsulation.
- If you stay in the same network (e.g. don’t go through a router, network mask says source and destination are in the same network) then MACs generally rule. It TCP/IP terms, the network access layer encapsulation is what is read and used by the switch
- If you cross network boundaries then the MAC moves you from source to router across the switched network and the network access encapsulation is removed. The internetworking layer comes into play when the IP addresses are updated with the router’s IP and the next hop destination IP from the routing table. Its re-wrapped in the network layer for crossing the next switched link. (Note: The Host-to-host layer kicks in then the router looks up the next hop– if using tcp/ip remember that the destination address stored in the encapsulated TCP packet is used by the router to determine the next hop.
Translation– listen to the teacher. This is really tough to explain without a picture. Also, go to the CISCO site– they have all kinds of free training on this stuff.

By: labnuke99

labnuke99 — Fri, 03 Sep 2010 20:12:04 +0000

The IP address information remains constant (with the exception of NAT as Mrdenny says) along the path. The only thing that changes are the MAC addresses but that is at the layer 2 part of the packet.