Ports & DNS

Ports

A port number is an addressing mechanism that identifies which application or service on a host machine should receive the data. IP gets data to the right machine; port gets it to the right program.

Types of Ports

Type	Range	Description
Well-known Ports	0 – 1023	Reserved for standard services
Registered Ports	1024 – 49151	Assigned to applications
Ephemeral / Private	49152 – 65535	Dynamically assigned to client sessions

Common Port Numbers DevOps Engineers Must Know

Port	Protocol	Service
22	TCP	SSH
25	TCP	SMTP (email)
53	UDP/TCP	DNS
80	TCP	HTTP
443	TCP	HTTPS
3306	TCP	MySQL
5432	TCP	PostgreSQL
6379	TCP	Redis
8080	TCP	Common app/proxy port
2379	TCP	etcd (used by Kubernetes)
6443	TCP	Kubernetes API server

DNS — Domain Name System

The DNS is a massive, distributed database that maps human-readable domain names to IP addresses.

Without DNS, you would need to type 142.250.191.46 instead of google.com.

DNS Resolution Flow

You type: google.com
    │
    ▼
Browser checks local cache
    │ (not found)
    ▼
OS checks /etc/hosts file
    │ (not found)
    ▼
Query sent to Recursive Resolver (your ISP or 8.8.8.8)
    │
    ▼
Recursive Resolver checks its cache
    │ (not found)
    ▼
Query to Root DNS Server (knows where .com lives)
    │
    ▼
Query to TLD Server (.com nameserver)
    │
    ▼
Query to Authoritative DNS Server (knows google.com)
    │
    ▼
Returns IP address → Browser connects

DNS Record Types

Record	Purpose	Example
A	Maps domain to IPv4 address	`google.com → 142.250.191.46`
AAAA	Maps domain to IPv6 address	`google.com → 2607:f8b0:...`
CNAME	Alias — one domain points to another	`www → google.com`
MX	Mail server for the domain	`mail.google.com`
TXT	Arbitrary text (used for verification, SPF)	`"v=spf1 include:..."`
NS	Nameservers responsible for the domain	`ns1.google.com`
PTR	Reverse DNS — IP to domain name	`142.250.191.46 → google.com`

TTL (Time to Live)

Each DNS record has a TTL — how long (in seconds) resolvers should cache it. Lower TTL = faster propagation during changes (useful during deployments), but more DNS queries.