Do routers change packet IP info at each hop?

5 pts.
Tags:
IP address
IP address management
MAC address
Routers
Routing
I'm taking classes for CCNA certification and think my professor may be wrong about this...

She says that at each hop, the router strips the source and destination IP and updates it to reflect its IP as the source and the next router's to be the destination. But it seems to me that if that's the case, the packet would not know where its final destination is nor would that destination know where to reply to.

I think she was confusing my question with MAC address updating at each hop (which we haven't covered in class yet). She also says that a source MUST know the MAC of the destination before sending communication, but I was under the impression that the source simply addressed communication to a final destination IP in the packet header and the first router's MAC in the frame header, and that each router along the path updated source and destination MAC addresses while leaving the IP addresses in the layer 3 header untouched.

Am I missing something or is she mistaken?

ASKED: September 3, 2010  6:15 PM
UPDATED: September 28, 2010  1:58 PM

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As far as I know the only time the IP address information would be removed from a TCP packet would be when the packet goes through a NAT or PAT router as it needs to change the private side information to public side information.

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  • Labnuke99
    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.
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  • GregBMe
    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.
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  • GregBMe
    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.
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  • TJEnger
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