Post
IP and networking cheat sheet
Addressing
| Concept | Meaning | Example |
|---|---|---|
| IPv4 address | 32-bit address identifying an interface | 192.0.2.10 |
| IPv6 address | 128-bit address identifying an interface | 2001:db8::10 |
| CIDR prefix | Number of network bits | /24 |
| Subnet mask | IPv4 form of a CIDR prefix | /24 = 255.255.255.0 |
| Default gateway | Router used for destinations outside the local subnet | 192.0.2.1 |
| Private IPv4 | Address space not routed on the public Internet | 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 |
| Documentation IPv4 | Safe examples for documents and labs | 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 |
| Loopback | Host refers to itself | 127.0.0.1, ::1 |
Ports and common services
| Service | Typical transport | Common port(s) | Role in voice |
|---|---|---|---|
| DNS | UDP/TCP | 53 | Resolves SIP servers, SRV records, and services |
| DHCP | UDP | 67/68 | Assigns addresses and network options |
| NTP | UDP | 123 | Keeps clocks aligned for logs, TLS, and troubleshooting |
| HTTP/HTTPS | TCP | 80/443 | Provisioning, APIs, and web administration |
| SIP | UDP/TCP/TLS | 5060/5061 | Call signalling |
| RTP/RTCP | UDP | Dynamic range | Audio/video media and quality reports |
| SSH | TCP | 22 | Secure administration |
Port numbers are conventions, not proof of what an application is carrying. Check the actual configuration and traffic; consult the IANA service-name and port registry.
Quick calculations
- Usable IPv4 hosts in a normal subnet:
2^(32-prefix) - 2; exceptions exist for/31and/32use. - Approximate codec bit rate on the wire: codec payload rate plus packet, transport, IP, link, and tunnel overhead.
- Every additional packet per second increases header overhead. Smaller packetization lowers latency but raises overhead.
Troubleshooting order
- Link and power: is the endpoint connected and negotiating correctly?
- Addressing: does it have the expected IP, gateway, DNS, and time?
- Reachability: can it resolve and reach the signalling service?
- Signalling: do registration and call setup complete?
- Media: are RTP packets flowing both ways?
- Quality: are loss, jitter, delay, or queueing degrading the call?