IP addresses and ports

A support ticket arrives: "The order page will not load." Your team lead reads it and asks one question. "Is it the address or the port?"

Every message on a network needs two numbers to arrive. One finds the computer. The other finds the program on it. In this lesson you learn to read both.

Every device on a network has an IP address. Laptops, phones, printers and servers all have one.

The common kind, IPv4, is four numbers with dots between them, like 192.168.1.23. Each number is from 0 to 255.

IPv6 addresses are longer and use letters too, like 2001:db8::10. They exist because the world ran short of IPv4 addresses.

address = "192.168.1.23"

# split cuts the text at every dot and gives a list of four parts
parts = address.split(".")
print(parts)

for part in parts:
    number = int(part)
    if number > 255:
        print(part, "is too big: each part is 0 to 255")
    else:
        print(part, "is fine")

# Change the address to "192.168.1.300", then press Run.

One server often runs several programs. It might run a website, a database and a remote login service at the same time.

A port is a number from 1 to 65535. It tells the server which program should get the message. Some numbers are used so widely that everybody knows them. A web server using HTTPS listens on port 443.

# A small copy of the list of well-known ports
printf "ssh 22\ndns 53\nhttp 80\nhttps 443\npostgres 5432\nmysql 3306\n" > ports.txt

# Which service listens on port 443?
grep " 443$" ports.txt

# Change 443 to 22, then press Run.

A socket is an address and a port written together: 203.0.113.10:443.

A connection has a socket at each end. The server's port is fixed and well known. The client picks a high port at random for each connection. That is how one laptop can have ten browser tabs open to the same shop.