Cheat sheet · Systems and cloud
Networking cheat sheet
Layers, ports, TCP and UDP, DNS records, IP ranges and the commands to check a connection.
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Cheat sheet
Networking
Layers, ports, TCP and UDP, DNS records, IP ranges and the commands to check a connection.
Layers (TCP/IP model)
Application- HTTP, DNS, SSH: what programs say
Transport- TCP, UDP: ports and delivery
Internet- IP: addresses and routing
Link- Ethernet, Wi-Fi: the local network
Well-known ports
22- SSH
53- DNS
80 / 443- HTTP / HTTPS
25 / 587- Email sending (SMTP)
3306 / 5432- MySQL / PostgreSQL
6379 / 27017- Redis / MongoDB
TCP and UDP
TCP- Reliable and in order; connection first (three-way handshake)
UDP- Fast, no promise of delivery; used for video, games, DNS
SYN, SYN-ACK, ACK- The three steps that open a TCP connection
DNS records
A / AAAA- Name to an IPv4 / IPv6 address
CNAME- A name that points to another name
MX- Where email goes
TXT- Free text, often for verification and email rules
NS- Which servers answer for the domain
TTL- How long an answer may be cached
IP addresses
10.0.0.0/8 172.16.0.0/12 192.168.0.0/16- Private ranges, not reachable from the internet
127.0.0.1- This machine (localhost)
/24- 256 addresses (254 usable)
/16 / /8- 65,536 addresses / about 16.7 million
0.0.0.0/0- Every address
Check a connection
ping <host>- Is it reachable?
dig <name>- Look up DNS records
traceroute <host>- The route the packets take
curl -v https://<host>- Watch the connection and TLS steps
ss -tulpn- Which program listens on which port
ip addr- Your machine's addresses
HTTPS in short
TLS handshake- Agree on keys and check the server's certificate
Certificate- Proves the site's identity; signed by a trusted authority
Remember
- "It works on my machine" is often DNS, a firewall or a wrong port. Check those three first.
- A DNS change can take up to the old TTL before everyone sees it.
Learn it properly
A cheat sheet helps you remember. These lessons explain the ideas step by step, with examples.