General

Checking ports and sockets

http://www.cyberciti.biz/tips/linux-investigate-sockets-network-connections.html

Check if you have a duplicate IP on your machine

arp-scan -I eth0 -l | grep -i dup

Check your IP from command line

curl ifconfig.me

Check what ports are listening on a server

netsat -ntpl

ss -l

Listening on the wire

Tcpdump eg port 80 and ip address

tcpdump -ni eth0 'dst 192.168.1.5 and tcp and port http'

Check for a virus on a network

tcpdump -tnli eth0 not port ssh and not port smtp and not domain and not http and not https and not imap

Other Tools

Iftop

wireshark

ntop