How to use the networking cheat sheets
- Browse common networking quick-references: OSI layers, port numbers, ICMP types, RFC 1918 ranges.
- Each card is print-friendly and includes the citation source.
- Save as PDF from your browser for offline study.
Cheat Sheets
Quick reference tables for subnetting, reserved IP ranges, and common port numbers.
Cheat Sheets
Quick reference tables for subnetting, reserved IP ranges, and common port numbers.
Subnetting Reference
| CIDR | Subnet Mask | # of /24s | Total IPs | Usable Hosts |
|---|
Reserved IPv4 Ranges
| Range | CIDR | Purpose |
|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | Private (RFC 1918) |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | Private (RFC 1918) |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | Private (RFC 1918) |
| 169.254.0.0 – 169.254.255.255 | 169.254.0.0/16 | Link-local (APIPA) |
| 127.0.0.0 – 127.255.255.255 | 127.0.0.0/8 | Loopback |
| 224.0.0.0 – 239.255.255.255 | 224.0.0.0/4 | Multicast |
| 240.0.0.0 – 255.255.255.254 | 240.0.0.0/4 | Reserved / Experimental |
Common Port Numbers
| 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 |
| 443 | TCP | HTTPS |
| 445 | TCP | SMB |
| 993 | TCP | IMAPS |
| 3389 | TCP/UDP | RDP |
Frequently asked questions
What are the 7 OSI model layers in order?
From bottom to top: Physical, Data Link, Network, Transport, Session, Presentation, Application. Mnemonic: 'Please Do Not Throw Sausage Pizza Away'.
Which TCP ports are most commonly probed?
Top targets in 2026 internet scans: 22 (SSH), 80 (HTTP), 443 (HTTPS), 3389 (RDP), 5900 (VNC), and 6379 (Redis). Lock down anything not strictly needed.
What are the RFC 1918 private IP ranges?
10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Plus the shared-address space 100.64.0.0/10 from RFC 6598 used by CGNAT.
Which ICMP types should I allow through a firewall?
At minimum: type 0 (echo reply), type 3 (destination unreachable, especially code 4 for Path MTU), type 8 (echo request) and type 11 (time exceeded). Blocking type 3 breaks MTU discovery.