What happens when you use a browser to access google.com

So this is just a quick recap of what I have learned about networking

First, you have to connect your computer to the local network. Your computer then will be assigned a local IP by a DHCP server (Dynamic Host Configuration Protocol), you cannot use that local IP to talk to the outside network. All the devices in the same local network will use the IP of the gateway to talk to the internet, sometimes it’s called public IP. The protocol to map from local address to public address is called NAT (Network Address Translation).

Google.com is a URL, the computer doesn’t understand those names, they made those up so the human can remember them easier.

Therefore, the first thing is to convert google.com to an IP address. The computer can do that using DNS protocol, each ISP will have a default DNS server, or you also can choose the DNS server that you want. So now you will have the IP of the server and be ready to make a connection.

Now, the client will make a TCP connection to the server using a “three ways handshake“. After that, if we use HTTPS, then right after TCP connection is established, TLS/SSL handshake will happen, the server will send a TLS/SSL Certification and it’s the public key to the client, the client then verify to see if the certificate is valid or not.

When the TCP connection is ready, the client can use HTTP/HTTPS protocol over TCP/IP protocol to talk to the server to request resources.

HTTP -> TCP -> IP -> Ethernet (PPP) -> Physical layer

TCP is a protocol to exchange messages between two processes, could be in the same or different machine. We usually see TCP come with IP, IP is another protocol to exchange messages between two hosts.

Bellow IP protocol could be Ethernet. So the IP package is transferred by Ethernet, and Ethernet doesn’t use an IP address, it uses MAC Address instead. So there is an ARP (Address Resolution Protocol) to get MAC addresses of surrounding hosts.

There is another protocol which is on the same layer (layer 2: data link layer) with Ethernet, it is PPP (point-to-point protocol). PPP is a dedicated connection between two nodes, unlike Ethernet which is in a multi-access network where multiple devices can share the same medium. PPP doesn’t need MAC address, address field in PPP frame is filled with 0xFF.

The lowest layer is physical layer, but I am not gonna talk about it, because it’s out of my knowledge.