In addition to IP and TCP, the Cisco TCP/IP implementation supports ARP, RARP, ICMP, Proxy ARP (in
which the router acts as an ARP server on behalf of another device), Echo, Discard, and Probe (an address
resolution protocol developed by Hewlett−Packard Company and used on IEEE 802.3 networks). Cisco
routers also can be configured to use the Domain Name System (DNS) when host name−to−address mappings
are needed.
IP hosts need to know how to reach a router. There are several ways this can be done:
· Add a static route in the host pointing to a router.
· Run RIP or some other IGP on the host.
· Run the ICMP Router Discovery Protocol (IRDP) in the host.
· Run Proxy ARP on the router.
Cisco routers support all of these methods.
Cisco provides many TCP/IP value−added features that enhance applications availability and reduce the total
cost of internetwork ownership. The most important of these features are described in the following section.

Access Restrictions:

Most networks have reasonably straightforward access requirements. To address these issues, Cisco
implements access lists, a scheme that prevents certain packets from entering or leaving particular networks.
An access list is a sequential list of instructions to either permit or deny access through a router interface
based on IP address or other criteria. For example, an access list could be created to deny access to a particular
resource from all computers on one network segment but permit access from all other segments. Another
access list could be used to permit TCP connections from any host on a local segment to any host in the
Internet but to deny all connections from the Internet into the local net except for electronic mail connections
to a particular designated mail host. Access lists are extremely flexible, powerful security measures and are
available not only for IP, but for many other protocols supported by Cisco routers.
Other access restrictions are provided by the Department of Defense−specified security extensions to IP.
Cisco supports both the Basic and the Extended security options as described in RFC 1108 of the IP Security
Option (IPSO). Support of both access lists and the IPSO makes Cisco a good choice for networks where
security is an issue.

Tunneling:

Cisco's TCP/IP implementation includes several schemes that allow foreign protocols to be tunneled through
an IP network. Tunneling allows network administrators to extend the size of AppleTalk and Novell IPX
networks beyond the size that their native protocols can handle.

IP Multicast:

The applications that use the TCP/IP protocol suite continue to evolve. The next set of applications on which a
lot of work is being done include those that use video and audio information. Cisco continues to be actively
involved with the Internet Engineering Task Force (IETF) in defining standards that will enable network
administrators to add audio and video applications to their existing networks. Cisco supports the Protocol
Independent Multicast (PIM) standard. In addition, Cisco's implementation provides interoperability with the IP multicasting (the ability to send IP datagrams to multiple nodes in a logical group) is an important building
block for applications such as video. Video teleconferencing, for example, requires the ability to send video
information to multiple teleconference sites. If one IP multicast datagram containing video information can be
sent to multiple teleconference sites, network bandwidth is saved and time synchronization is closer to
optimal.