How a name becomes an address

A new colleague types intranet.example.com and sees "server not found". You type 192.168.1.50 on the same laptop and the page opens.

So the server is fine. The problem is turning the name into the number. That job belongs to a system called DNS, and it is behind almost every request you make.

Your laptop does not know where shop.example.com is. It asks a resolver. In an office, that is often the router or a company server.

If the resolver does not know either, it asks in steps. First a root server, which points to the servers for .com. They point to the servers for example.com. Those give the answer, and the resolver passes it back to your laptop.

# A tiny copy of what a name server knows
records = {
    "shop.example.com": "203.0.113.10",
    "mail.example.com": "203.0.113.25",
    "intranet.example.com": "192.168.1.50",
}

name = "shop.example.com"

if name in records:
    print(name, "is at", records[name])
else:
    print(name, "has no record: the browser shows 'server not found'")

# Change name to "payroll.example.com", then press Run.

A name server keeps records about each name. Each record has a type.

An A record gives an IPv4 address. An AAAA record gives an IPv6 address. A CNAME says "this name is really that name", so the lookup carries on. An MX record says which server takes the email for a domain.

When you set up a new service at work, you often ask for one of these.

Asking the whole chain every time would be slow. So a resolver keeps each answer for a number of seconds. That number is the TTL.

Below is real output from dig, a tool engineers use to ask DNS directly. The 300 on each answer line is the TTL: five minutes. The last line shows which resolver answered.

$ dig shop.example.com

;; QUESTION SECTION:
;shop.example.com.          IN  A

;; ANSWER SECTION:
shop.example.com.   300  IN  CNAME  web.example.com.
web.example.com.    300  IN  A      203.0.113.10

;; Query time: 24 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)

# A hosts file: one address and one name on each line
printf "192.168.1.50 intranet.example.com\n192.168.1.60 wiki.example.com\n" > hosts

# Find the address for the wiki
grep "wiki.example.com" hosts | awk '{print $1}'

# Change wiki to intranet, then press Run.