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Arsenal·Networking

Networking Cheat Sheet

Subnetting, CIDR, common ports, and the OSI model on one page—quick reference for engineers and interview prep.

2 min read·By Arun R Kaushik

OSI Model

Layer Name Examples
7 Application HTTP, DNS, SMTP, FTP
6 Presentation TLS, encoding, compression
5 Session sessions, sockets
4 Transport TCP, UDP
3 Network IP, ICMP, routing
2 Data Link Ethernet, switches, MAC
1 Physical cables, fiber, radio

CIDR Quick Reference

CIDR Subnet Mask Hosts
/24 255.255.255.0 254
/25 255.255.255.128 126
/26 255.255.255.192 62
/27 255.255.255.224 30
/28 255.255.255.240 14
/29 255.255.255.248 6
/30 255.255.255.252 2

Usable hosts = 2^(32 - prefix) - 2

Private IP Ranges (RFC 1918)

  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

Common Ports

Port Protocol Service
20/21 TCP FTP
22 TCP SSH
23 TCP Telnet
25 TCP SMTP
53 TCP/UDP DNS
67/68 UDP DHCP
80 TCP HTTP
110 TCP POP3
123 UDP NTP
143 TCP IMAP
161/162 UDP SNMP
179 TCP BGP
389 TCP LDAP
443 TCP HTTPS
445 TCP SMB
514 UDP Syslog
3389 TCP RDP

TCP vs UDP

TCP UDP
Connection Connection-oriented Connectionless
Reliability Guaranteed delivery, ordering Best effort
Overhead Higher (handshake, ACKs) Lower
Use cases HTTP, SSH, databases DNS, VoIP, streaming

TCP Three-Way Handshake

  1. Client → Server: SYN
  2. Server → Client: SYN-ACK
  3. Client → Server: ACK

VLSM Quick Tip

When subnetting for multiple sized networks, allocate the largest subnets first, then carve smaller ones from the remaining space to avoid fragmentation.

Useful Commands

# Show interfaces and IPs
ip addr show

# Show routing table
ip route show

# Trace path to a host
traceroute example.com

# DNS lookup
dig example.com
nslookup example.com

# Test a TCP port
nc -zv example.com 443