TCP/IP and DNS — Reaching the Server
Before the BookNook app can ask for a single book, it has to find the server and open a line to it. Two old, quiet systems do that work on every request: DNS turns the name into an address, and TCP/IP carries the bytes there and back. When an API "is down" but the server is fine, the fault is often in one of these two.
The rule
DNS finds the address; IP delivers the packets; TCP keeps them in order.
DNS (Domain Name System) is the internet's phone book. The app knows the name api.booknook.example, but networks route by number. A DNS lookup returns the number:
$ dig +short api.booknook.example A
203.0.113.10- An A record maps a name to an IPv4 address; an AAAA record maps it to an IPv6 address.
- A CNAME record says "this name is an alias of that name".
- Each answer has a TTL (time to live): how many seconds resolvers may cache it. A long TTL means a changed address takes longer to reach everyone.
IP (Internet Protocol) moves small packets from one address to another, hop by hop through routers. It does its best, but packets may arrive late, twice, out of order or not at all.
TCP (Transmission Control Protocol) sits on top of IP and turns that unreliable stream of packets into a reliable, ordered conversation. It numbers every byte, re-sends anything lost and opens with a three-step handshake: SYN, SYN-ACK, ACK. A port picks which program on the machine gets the data: 443 for HTTPS, 80 for plain HTTP.
On top of TCP, HTTPS adds TLS encryption, and on top of that runs HTTP, the language of the API itself.
The shortcut
Walk the layers from the name down, then back up: name → address → connection → encryption → request.
Name to address
DNS resolves
api.booknook.exampleto203.0.113.10, using a cached answer if the TTL has not run out.
When something fails, the layer tells you who to blame: "could not resolve host" is DNS, "connection refused" or a timeout is TCP or the network, and a 500 error is the API itself.
Drills
Answer fast — trust the rule.
What does a DNS lookup for api.booknook.example return?
Packets can arrive out of order on the internet. Which protocol puts them back in order for the API?
A client error says "could not resolve host api.booknook.example". Where is the problem?
BookNook moves to a new server, but some users reach the old address for hours. What most likely explains it?
Remember
- DNS finds the address; IP delivers packets; TCP keeps them reliable and in order.
- Shortcut: name → address → connection → encryption → request.
- Trap: blaming the API for a DNS or network failure; the error message names the layer.