| A | B |
|---|
| NAT | Network Address Translation |
| Why use NAT | NAT is a mechanism for conserving registered IP addresses in large networks and simplifying IP addressing management tasks |
| DHCP | Dynamic Host Configuration Protocol |
| Private addressing | RFC 1918 sets aside three blocks of private IP addresses. They are one Class A address, 16 Class B addresses, and 256 Class C addresses |
| Private address numbers - CIDR | 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16 |
| Cisco defined NAT terms | Inside local address; Inside global address ; Outside local address; Outside global address |
| Inside local address | The IP address assigned to a host on the inside network. The address is usually not an IP address assigned by the Network Information Center (NIC) or service provider. This address is likely to be an RFC 1918 private address. |
| Inside global address | A legitimate IP address assigned by the NIC or service provider that represents one or more inside local IP addresses to the outside world |
| Outside local address | The IP address of an outside host as it is known to the hosts on the inside network. |
| Outside global address | The IP address assigned to a host on the outside network. The owner of the host assigns this address. |
| A NAT enabled device typically operates at | border of a stub network. A stub network is a network that has a single connection to its neighbor network. |
| Overloading, or Port Address Translation (PAT), | maps multiple private IP addresses to a single public IP address. Multiple addresses can be mapped to a single address because each private address is tracked by a port number. |
| PAT uses | unique source port numbers on the inside global IP address to distinguish between translations. PAT will attempt to preserve the original source port. If this source port is already used, PAT will assign the first available port number starting from the beginning of the appropriate port group 0-511, 512-1023, or 1024-65535 |
| Static Translation | Static translations are entered directly into the configuration and are always in the translation table |
| Dynamic Translation | The access list must permit only those addresses that are to be translated. |
| Overloading | Overloading is configured in two ways depending on how public IP addresses have been allocated. An ISP can allocate a network only one public IP address, and this is typically assigned to the outside interface which connects to the ISP. Another way of configuring overload is if the ISP has given one or more public IP addresses for use as a NAT pool. |
| Verifying PAT configuration | Once NAT is configured, use the clear and show commands to verify that it is operating as expected. Alternatively, use the show run command and look for NAT, access list, interface, or pool commands with the required values |
| Troubleshooting NAT and PAT configuration | When IP connectivity problems in a NAT environment exist, it is often difficult to determine the cause of the problem. Many times NAT is mistakenly blamed, when in reality there is an underlying problem. |
| NAT has several advantages | (1) NAT conserves the legally registered addressing scheme by allowing the privatization of intranets. (2) 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. (3) 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 has some disadvantages | 1. NAT increases delay; 2. 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. 3. NAT also forces some applications that use IP addressing to stop functioning because it hides end-to-end 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. |
| 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. |
| The four basic IP parameters | IP address; Gateway address; Subnet mask; DNS server address |
| two primary differences between DHCP and BOOTP | (1) 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. (2) DHCP provides the mechanism for a client to gather other IP configuration parameters, such as WINS and domain name. |
| Major DHCP features | Automatic allocation; Manual allocation; Dynamic allocation |
| 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. |
| DHCP operation | 1.client locates a DHCP server by sending a broadcast called a DHCPDISCOVER; 2. the DHCP server offers the client IP configuration information in the form of a unicast DHCPOFFER. The DHCPOFFER is a proposed configuration that may include IP address, DNS server address, and lease time. 3. If the client finds the offer agreeable, it will send another broadcast, a DHCPREQUEST, specifically requesting those particular IP parameters; 4. The server that receives the DHCPREQUEST makes the configuration official by sending a unicast acknowledgment, the DHCPACK |
| Configuring DHCP | 1. DHCP server requires that the administrator define a pool of addresses; 2. Specify the range of the address pool. |
