intranets.

Increases the flexibility of connections to the public network. Multiple pools, backup pools, and load balancing pools can be implemented to assure reliable public network connections.

Consistency of the internal network addressing scheme. On a network without private IP addresses and NAT, changing public IP addresses requires the renumbering of all hosts on the existing network. The costs of renumbering hosts can be significant. NAT allows the existing scheme to remain while supporting a new public addressing scheme.

NAT is not without drawbacks. Enabling address translation will cause a loss of functionality, particularly with any protocol or application that involves sending IP address information inside the IP payload. This requires additional support by the NAT device. NAT increases delay. Switching path delays are introduced because of the translation of each IP address within the packet headers. Performance may be a consideration because NAT is currently accomplished by using process switching. The CPU must look at every packet to decide whether it has to translate it. The CPU must alter the IP header, and possibly alter the TCP header. One significant disadvantage when implementing and using NAT is the loss of end-to-end IP traceability. It becomes much more difficult to trace packets that undergo numerous packet address changes over multiple NAT hops. Hackers who want to determine the source of a packet will find it difficult to trace or obtain the original source or destination address. NAT also forces some applications that use IP addressing to stop functioning because it hides end-to-end IP addresses. Applications that use physical addresses instead of a qualified domain name will not reach destinations that are translated across the NAT router. Sometimes, this problem can be avoided by implementing static NAT mappings. Cisco IOS NAT supports the following traffic types:

• ICMP
• File Transfer Protocol (FTP), including PORT and PASV commands
• NetBIOS over TCP/IP, datagram, name, and session services
• RealNetworks' RealAudio
• White Pines' CUSeeMe
• Xing Technologies' StreamWorks
• DNS "A" and "PTR" queries
• H.323/Microsoft NetMeeting, IOS versions 12.0(1)/12.0(1)T and later
• VDOnet's VDOLive, IOS versions 11.3(4)11.3(4)T and later
• VXtreme's Web Theater, IOS versions 11.3(4)11.3(4)T and later
• IP Multicast, IOS version 12.0(1)T with source address translation only

Cisco IOS NAT does not support the following traffic types:

• Routing table updates
• DNS zone transfers
• BOOTP
• talk and ntalk protocols
• Simple Network Management Protocol (SNMP)

Interactive Media Activity Checkbox: Issues with NAT When the student has completed this activity, the student will be able to identify issues with the use of NAT. Web Links Verifying NAT Operation and Basic NAT Troubleshooting http://www.cisco.com/en/US/tech/
tk648/tk361/technologies_tech_note
09186a0080094c32.shtml

---

Content 1.2 DHCP 1.2.1 Introducing DHCP Dynamic Host Configuration Protocol (DHCP) works in a client/server mode. DHCP enables DHCP clients on an IP network to obtain their configurations from a DHCP server. Less work is involved in managing an IP network when DHCP is used. The most significant configuration option the client receives from the server is its IP address. The DHCP protocol is described in RFC 2131. A DHCP client is included in most modern operating systems including the various Windows operating systems, Novell Netware, Sun Solaris, Linux, and MAC OS. The client requests addressing values from the network DHCP server. This server manages the allocation of the IP addresses and will answer configuration requests from clients. The DHCP server can answer requests for many subnets. DHCP is not intended for configuring routers, switches, and servers. These type of hosts all need to have static IP addresses. DHCP works by providing a process for a server to allocate IP information to clients. Clients lease the information from the server for an administratively defined period. When the lease expires the client must ask for another address, although the client is typically reassigned the same address. Administrators typically prefer a network server to offer DHCP services because these solutions are scalable and relatively easy to manage. Cisco routers can use a Cisco IOS feature set, Easy IP, to offer an optional, full-featured DHCP server. Easy IP leases configurations for 24 hours by default. This is useful in small offices and home offices where the home user can take advantage of DHCP and NAT without having an NT or UNIX server. Administrators set up DHCP servers to assign addresses from predefined pools. DHCP servers can also offer other information, such as DNS server addresses, WINS server addresses, and domain names. Most DHCP servers also allow the administrator to define specifically what client MAC addresses can be serviced and automatically assign them the same IP address each time. DHCP uses UDP as its transport protocol. The client sends messages to the server on port 67. The server sends messages to the client on port 68. Web Links DHCP Overview http://support.baynetworks.com/library/ tpubs/html/ router/soft1200/ 117362AA/A_29.HTM

---

Content 1.2 DHCP 1.2.2 BOOTP and DHCP differences The Internet community first developed the BOOTP protocol to enable configuration of diskless workstations. BOOTP was originally defined in RFC 951 in 1985. As the predecessor of DHCP, BOOTP shares some operational characteristics. Both protocols are client/server based and use UDP ports 67 and 68. Those ports are still known as BOOTP ports.The four basic IP parameters:

• IP address
• Gateway address
• Subnet mask
• DNS server address

BOOTP does not dynamically allocate IP addresses to a host. When a client requests an IP address, the BOOTP server searches a predefined table for an entry that matches the MAC address for the client. If an entry exists, then the corresponding IP address for that entry is returned to the client. This means that the binding between the MAC address and the IP address must have already been configured in the BOOTP server. There are two primary differences between DHCP and BOOTP:

• DHCP defines mechanisms through which clients can be assigned an IP address for a finite lease period. This lease period allows for reassignment of the IP address to another client later, or for the client to get another assignment, if the client moves to another subnet. Clients may also renew leases and keep the same IP address.
• DHCP provides the mechanism for a client to gather other IP configuration parameters, such as WINS and domain name.

Web Links BootP Relay Agent Overview http://support.baynetworks.com/library/ tpubs/html/ router/ soft1200/117362AA/ A_28.HTM#MARKER-9-90

---

Content 1.2 DHCP 1.2.3 Major DHCP features There are three mechanisms used to assign an IP address to the client:

• Automatic allocation – DHCP assigns a permanent IP address to a client.
• Manual allocation – The IP address for the client is assigned by the administrator. DHCP conveys the address to the client.
• Dynamic allocation – DHCP assigns, or leases, an IP address to the client for a limited period of time.

The focus of this section is the dynamic allocation mechanism. Some of the configuration parameters available are listed in IETF RFC 1533:

• Subnet mask
• Router
• Domain Name
• Domain Name Server(s)
• WINS Server(s)

The DHCP server creates pools of IP addresses and associated parameters. Pools are dedicated to an individual logical IP subnet. This allows multiple DHCP servers to respond and IP clients to be mobile. If multiple servers respond, a client can choose only one of