routing rudiments...
given: hosts are directly connected, host_A pings host_B
with a nod to lammle, odom, et al
1) host_A creates an echo request for host_B (ICMP)
2) ICMP hands the request to IP, which makes a packet that contains the IP source address, the IP destination address and a protocol field
3) IP determines whether the destination is a local or remote network
4) in this case it is remote, so the packet needs to go to the remote network determined by host_A’s OS (default gateway 172.16.10.1)
5) the MAC address of 172.16.10.1 (E0) is known by host_A. this supposes the arp cache has resolved E0’s IP address to a hardware address. (the output of arp -a will display E0’s MAC address) the packet will be framed by the datalink layer and shipped to the MAC address of E0. hosts only communicate with MAC addresses on the local network
6) if not already resolved, the host sends an arp broadcast to the local network. Upon receipt of the resolution, the host cache’s the MAC address of E0 and encapsulates the packet in a frame with control information.
In both the above cases (hardware address known, hardware address resolved) the framed destination MAC address (E0) and source MAC address (host_A), an ether-type field (network layer IP), the packet, and FCS (frame check sequence which holds the result of a cyclic redundancy check CRC) are shipped to the MAC address of the default gateway, not the MAC address of the remote host
7) the frame thusly prepared, is ready to traverse the physical layer bit by bit
8) at the router, the packet is taken from the frame and off loaded to IP. The remainder of the frame is discarded.
a good place to jump
a good place to jump
