Common Ports
20-21 FTP (Data/Control) 22 SSH 23 Telnet 25 SMTP 53 DNS 67-68 DHCP (Server/Client) 80 HTTP 110 POP3 143 IMAP 443 HTTPS 3306 MySQL 5432 PostgreSQL 6379 Redis 8080 HTTP Alt Subnetting Quick Reference
/32 = 1 host 255.255.255.255 /31 = 2 hosts 255.255.255.254 (P2P) /30 = 4 hosts (2 usable) 255.255.255.252 /29 = 8 hosts (6 usable) 255.255.255.248 /28 = 16 hosts (14 usable) 255.255.255.240 /27 = 32 hosts (30 usable) 255.255.255.224 /26 = 64 hosts (62 usable) 255.255.255.192 /25 = 128 hosts (126 usable) 255.255.255.128 /24 = 256 hosts (254 usable) 255.255.255.0 /16 = 65,536 hosts 255.255.0.0 /8 = 16,777,216 hosts 255.0.0.0 Private IP Ranges (RFC 1918)
10.0.0.0/8 Class A (16M hosts) 172.16.0.0/12 Class B (1M hosts) 192.168.0.0/16 Class C (65K hosts) 169.254.0.0/16 Link-Local (APIPA) 127.0.0.0/8 Loopback Diagnostic Commands
ping <host> Test connectivity ping -c 4 <host> Ping 4 times traceroute <host> Trace route (Linux) tracert <host> Trace route (Windows) mtr <host> Combined ping/traceroute nslookup <domain> DNS lookup dig <domain> DNS query dig +short <domain> Short DNS answer host <domain> Simple DNS lookup whois <domain> Domain info Network Info
ip addr Show IP addresses (Linux) ip route Show routes (Linux) ip link Show interfaces (Linux) ifconfig Show interfaces (legacy) netstat -tuln Listening ports ss -tuln Listening ports (modern) arp -a ARP table ip neigh ARP table (modern) Connection Testing
nc -zv <host> <port> Test TCP port nc -zuv <host> <port> Test UDP port telnet <host> <port> Test TCP connection curl -I <url> HTTP headers curl -v <url> Verbose HTTP wget --spider <url> Check URL exists Packet Capture
tcpdump -i eth0 Capture on interface tcpdump -i eth0 port 80 Capture port 80 tcpdump -i eth0 host 10.0.0.1 Capture host tcpdump -w capture.pcap Write to file tcpdump -r capture.pcap Read from file tcpdump -n No DNS resolution Firewall (iptables)
iptables -L -n -v List rules iptables -A INPUT -p tcp --dport 22 -j ACCEPT Allow SSH iptables -A INPUT -j DROP Drop all input iptables -F Flush all rules iptables-save > rules.txt Save rules iptables-restore < rules.txt Restore rules