Notes

Let’s bring in two networks, network A and B. Network A will have a network space of 4.5.9.0/24 and network B will have a network space of 1.2.3.0/24. These networks then connect to a router with two interfaces having IP addresses of 9.3.4.1 on A, and 145.2.46.1 for B.

Let’s introduce a client running off hardware which is a part of network A. The IP 4.5.9.22 wants to access a website with an IP address of 143.6.43.22. If the user on this client opens a browser and inputs this IP in the address bar, the browser will communicate with local networking and attempt to establish a TCP connection with the server via port 80. This step enables networking stacking to check its own subnet to tell the user 143.6.43.22 is on another network and that the data will have to travel through a gateway with the interference of network A. The client then checks the ARP table to get the MAC address. If the MAC address is not found, it will send a request to ARP and continues by having any device connected on 9.3.4.1 receive this ARP request. This then goes to the router which then responds with Its own MAC address to the client device. This creates a gateway address for the client. The OS then opens an ephemeral port and opens a socket for a connection to the browser to go through that port. This is the destination port.

The network stack then starts to create the TCP segment using the source and destination port. A sequence number is chosen to fill in the sequence number field as well. Then, the SYN flag is set, and a checksum for the segment is calculated and written to it’s appropriate field. This TCP segment is then passed along to the IP layer of the networking stack. This creates an IP header. Inside this IP header is the source IP, the destination IP, and a TTL of 64. This segment is then set to the payload for the datagram. The checksum is calculated and valued. This needs to get to the gateway, so an Ethernet datagram is created. The source, destination, and MAC address fields then get filled and the datagram is inserted as payload of the Ethernet frame and then checksum is calculated and valued.

Then, the physical layer takes this information and passes it through a wire connecting the client and the network switch. The switch takes this information because it knows the location of the MAC address and then it is passed to the router. After this, the router checks the information and performs a checksum to make sure the information matches and compares the two. After this, it removes the Ethernet frame with the remaining datagram and performs another checksum to ensure the data matches. It then searches the routing table to find the destination IP datagram which can be taken by the Ethernet frame with the destination and source ports and the MAC address. This is then sent to the server which connects the client to the web browser