Routing

[Commander_Keen]

If PC A on Subnet A pings PC B on Subnet B and has to go through 5 routers, how many times does the Source Mac address change.
I am under the understanding that once the TCP header leaves the PC A it (TCP Header/packet ) gets updated as it is leaving the exit interface of the first 1 router. But what happes, when it reaches to the 5th router.
Does the header ( Source Mac Address) change when it goes from Router to router?

[DonB]

The mac address is layer 2 and TCP/IP is layers 3 and 4.

The layer 2 uses the mac address for sending packets on the local link only therefore, both the source mac and the destination mac will change on each link the packet travels. The source mac will be the mac address of the exit interface and the destination mac will be the mac address of the next destination device either a client or inbound interface of the next hop router

[Commander_Keen]

So if a packet passes through 5 routers, then does the source change 5 times?

[Fuzz]

PC A - Router A = first write of src/dst MAC
Router A - Router B = first rewrite of src/dst
Router B - Router C = second rewrite of src/dst
Router C - Router D = third rewrite of src/dst
Router D - Router E = fourth rewrite of src/dst
Router E - PC B = fifth rewrite of src/dst

So you see the MAC address gets written 6 times, but changes 5 times.

[DonB]

Ethernet uses the mac address to send frames between device on a network LINK.

PC A ---> Router A Source mac is your pc Destination mac is Router A
Router A --> Router B Source mac is Router A Desintation mac is Router B
Router B --> Router C Source mac is Router B Desintation mac is Router C
Router C --> Router D Source mac is Router C Desintation mac is Router D
Router D --> Router E Source mac is Router D Desintation mac is Router E
Router E --> PC B Source mac is Router E Desintation mac is PC B

[Agnel]

Originally Posted by DonB

Ethernet uses the mac address to send frames between device on a network LINK.

PC A ---> Router A Source mac is your pc Destination mac is Router A
Router A --> Router B Source mac is Router A Desintation mac is Router B
Router B --> Router C Source mac is Router B Desintation mac is Router C
Router C --> Router D Source mac is Router C Desintation mac is Router D
Router D --> Router E Source mac is Router D Desintation mac is Router E
Router E --> PC B Source mac is Router E Desintation mac is PC B

Just to add some extra details.

When the frame leaves the router, the source address in the frame is that of the mac address of the exit interface of the router. The destination mac address is always of that of the next immediate device. (Usually the receiving interface of the next router).

The Source IP and Destination IP never changes.

[Commander_Keen]

GOT IT !!!!!

thanks guys for your help with this issue.

[anurag007]

Originally Posted by Agnel

The Source IP and Destination IP never changes.

Wat abt NAT !!!