Question

1. How does Wireshark know the manufacturer of the device sending a packet?

2. Why is a random interval added to the CSMA backoff and try again protocol?

3. Where are the routing tables for the backbone routers of the Internet calculated?

Answer 1

1. To completely understand wireshark you have to understand the usage of hubs and switches forwarding Ethernet traffic in a LAN. When we run Wireshark on our computer and the PC is connected to a switch, Wireshark will only see the packets from the PC to the switch. It cannot see other traffic over the network as a switch will not allow that. On the other hand, a hub will flood traffic out of every port.

2. Backoff and try again protocol algorithm is used in ethernet to schedule retransmissions after collisions. For a collision between two stations, they may restart transmission as soon as they can after collision and it may further lead to a collision. So an infinite loop is formed. So to prevent such scenario random interval is assigned so that it won't result in collisions.

3. The backbone routers of the Internet Standard Protocol constitute of the default-free zone. The default-free zone constitutes of no default routes, every route that is announced on the internet is carried in the routers' router table. So, it simply comes down to counting the table entries. If you want to count yourself, then arrange a BGP peering session with the ISP of yours and have them send you all routes.