What is Nemesis?
Nemesis is a command-line network packet injection utility for UNIX-like and Windows systems. You might think of it as an EZ-bake packet oven or a manually controlled IP stack. With Nemesis, it is possible to generate and transmit packets from the command line or from within a shell script. Nemesis is developed and maintained by Jeff Nathan

Nemesis for Windows systems
latest version: nemesis-1.4beta3.zip Build 22 (ChangeLog) (CHECKSUM) [Jun 29 2003]
supported protocols: ARP, DNS, ETHERNET, ICMP, IGMP, IP, OSPF, RIP, TCP and UDP
supported platforms: Windows 9x, Windows NT, Windows 2000, Windows XP

Examples
nemesis tcp -v -S 192.168.1.1 -D 192.168.2.2 -fSA -y 22 -P foo
Send TCP packet (SYN/ACK) with payload from file 'foo' to target's ssh port from 192.168.1.1 to 192.168.2.2. (-v allows a stdout visual of current injected packet)

nemesis udp -v -S 10.11.12.13 -D 10.1.1.2 -x 11111 -y 53 -P bindpkt
send UDP packet from 10.11.12.13:11111 to 10.1.1.2's name-service port with a payload read from a file 'bindpkt'. (again -v is used in order to see confirmation of our injected packet)

nemesis icmp -S 10.10.10.3 -D 10.10.10.1 -G 10.10.10.3 -qR
send ICMP REDIRECT (network) packet from 10.10.10.3 to 10.10.10.1 with preferred gateway as source address. Here we want no output to go to stdout - which would be ideal as a component in a batch job via a shell script.

nemesis arp -v -d ne0 -H 0:1:2:3:4:5 -S 10.11.30.5 -D 10.10.15.1
send ARP packet through device 'ne0' (eg. my OpenBSD pcmcia nic) from hardware source address 00:01:02:03:04:05 with IP source address 10.11.30.5 to destination IP address 10.10.15.1 with broadcast destination hardware address. In other words, who-has the mac address of 10.10.15.1, tell 10.11.30.5 - assuming 00:01:02:03:04:05 is the source mac address of our 'ne0' device.