Tools
Two interactive tools for the topics most worth practising by hand.
Subnet calculator
Drag the prefix slider and check your own working before you look at the answer. Subnetting is the most tested and most commonly failed topic in the whole process.
IPv4 subnet calculator
/24| Subnet mask | 255.255.255.0 |
|---|---|
| Wildcard | 0.0.0.255 |
| Network address | 192.168.1.0 |
| First usable host | 192.168.1.1 |
| Last usable host | 192.168.1.254 |
| Broadcast | 192.168.1.255 |
| Next network | 192.168.2.0 |
| Usable hosts | 254 (of 256) |
Port reference
Filter by the area you are revising.
Port reference — 35 of 35
| Port | Proto | Service | Why support cares |
|---|---|---|---|
20 | TCP | FTP data | Passive FTP. Blocked by most firewalls. |
21 | TCP | FTP control | Cleartext credentials — avoid. |
22 | TCP | SSH | Linux/Unix remote administration. |
23 | TCP | Telnet | Cleartext. Your JD names it as a test tool. |
25 | TCP | SMTP | Outbound mail from a mail client. |
53 | TCP/UDP | DNS | UDP for lookups, TCP for zone transfers. |
67 | UDP | DHCP server | Offers and acknowledgements. |
68 | UDP | DHCP client | Discover and Request. |
69 | UDP | TFTP | Used during PXE network boot. |
80 | TCP | HTTP | Unencrypted web. |
88 | UDP/TCP | Kerberos | AD authentication ticket exchange. |
123 | UDP | NTP | Time sync — Kerberos fails without it. |
137 | UDP | NetBIOS name | Legacy Windows name service. |
139 | TCP | NetBIOS session | Legacy file sharing. |
143 | TCP | IMAP | Mail retrieval. |
161 | UDP | SNMP | Network monitoring polling. |
162 | UDP | SNMP trap | Unsolicited device alerts. |
389 | TCP/UDP | LDAP | AD directory queries. |
443 | TCP | HTTPS | Also RDP-over-Gateway, SQL Server TLS. |
464 | TCP/UDP | Kerberos password change | |
500 | UDP | IKE (IPsec) | VPN negotiation phase 1. |
514 | UDP | Syslog | Centralised log shipping. |
636 | TCP | LDAPS | LDAP over TLS. |
3389 | TCP/UDP | RDP | Remote Desktop. Almost guaranteed in a support call. |
4500 | UDP | NAT-T / IPsec | VPN when UDP 500 is blocked. |
3268 | TCP | Global Catalog | AD global catalogue search. |
3269 | TCP | Global Catalog TLS | |
5985 | TCP | WinRM (HTTP) | PowerShell Remoting. |
5986 | TCP | WinRM (HTTPS) | Remoting over TLS. |
1433 | TCP | MS SQL Server | |
3306 | TCP | MySQL | |
5432 | TCP | PostgreSQL | |
27017 | TCP | MongoDB | |
8080 | TCP | HTTP alternate | Proxies, Tomcat. |
8443 | TCP | HTTPS alternate |
Practise active recall
Checklist items persist in your browser between visits.
Readiness check
0 / 6 complete
Why these tools exist
They are here to check you, not to replace practice. The subnet calculator is useful for verifying arithmetic, but the interview requires you to do it from memory on paper. Use this to correct mistakes, then close it and drill.
See IP Addressing for the method and the common mistakes.
Go deeper
| Topic | Page |
|---|---|
| Subnetting method and worked examples | IP Addressing |
| netstat, telnet, nmap, Test-NetConnection | Port Testing |
| Full port table with reasons | Port Cheatsheet |
| Every diagnostic command | Command Reference |