iptables is extensible, meaning that both the kernel and the iptables tool can be extended to provide new features.
Kernel extensions normally live in the kernel module subdirectory,such as /lib/modules/2.4.0-test10/kernel/net/ipv4/netfilter. They are demand loaded if your kernel was compiled with CONFIG_KMOD set, so you should not need to manually insert them.
Extensions to the iptables program are shared libraries which usually live in /usr/local/lib/iptables/, although a distribution would put them in /lib/iptables or /usr/lib/iptables.
Extensions come in two types:
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new targets, and new matches (we'll talk about new targets a little later). Some protocols automatically offer new tests: currently these are TCP, UDP and ICMP as shown below.
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