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IP addresses, subnet masks, default gateways, and DNS servers can be assigned statically or dynamically. You should statically assign most shared network systems, such as routers and servers, but dynamically assign most client systems. This section covers the protocols you use to dynamically assign IP address parameters to a host, which are the Bootstrap Protocol (BOOTP) and the Dynamic Host Configuration Protocol (DHCP). This section also covers DNS and ARP, which are two significant protocols in IP networks. DNS maps domain names to IP addresses, and ARP resolves IP addresses to MAC addresses. These protocols are important in TCP/IP networks because they simplify the methods of address assignment and resolution.
Static and Dynamic IP Address Assignment
Assign the IP addresses of routers, switches, printers, and servers statically. You need to manage and monitor these systems, so you must access them via a stable IP address.
You should dynamically assign end-client workstations to reduce the configuration tasks required to connect these systems to the network. When you assign client workstation characteristics dynamically, the system automatically learns which network segment it is assigned to and how to reach its default gateway as the network is discovered. One of the first methods used to dynamically assign IP addresses was BOOTP. The current method to assign IP addresses is DHCP.
BOOTP
The basic BOOTP was first defined in RFC 951. It has been updated by RFC 1497 and RFC 1542. It is a protocol that allows a booting host to configure itself by dynamically obtaining its IP address, IP gateway, and other information from a remote server. You can use a single server to centrally manage numerous network hosts without having to configure each host independently.
BOOTP is an application-layer protocol that uses UDP/IP for transport. The BOOTP server port is UDP Port 67. The client port is UDP Port 68. Clients send BOOTP requests to the BOOTP server, and the server responds to UDP Port 68 to send messages to the client. The destination IP of the BOOTP requests uses the all-hosts address (255.255.255.255), which the router does not forward. If the BOOTP server is one or more router hops from the subnet, you must configure the local default gateway router to forward the BOOTP requests.
BOOTP requires that you build a MAC-address-to-IP-address table on the server. You must obtain every device's MAC address, which is a time-consuming effort. BOOTP has been replaced by the more sophisticated DHCP.
DHCP
DHCP provides a way to dynamically configure hosts on the network. Based on BOOTP, it is defined in RFC 2131 and adds the capability of reusing network addresses and additional configuration options. DHCP improves on BOOTP by using a "lease" for IP addresses and providing the client with all the IP configuration parameters needed to operate in the network.
DHCP servers allocate network addresses and deliver configuration parameters dynamically to hosts. With DHCP, the computer can obtain its configuration information—IP address, subnet mask, IP default gateway, DNS servers, WINS servers, and so on—when needed. DHCP also includes other optional parameters that you can assign to clients. The configuration information is managed centrally on a DHCP server.
Routers act as relay agents by passing DHCP messages between DHCP clients and servers. Because DHCP is an extension of BOOTP, it uses the message format defined in RFC 951 for BOOTP. It uses the same ports as BOOTP: DHCP servers use UDP Port 67, and DHCP clients use UDP Port 68. Because of these similarities, the configuration to support DHCP in the routers is the same described for BOOTP.
DHCP supports permanent allocation, in which the DHCP server assigns an IP address to the client and the IP address is never reallocated to other clients. With a lease, DHCP can also assign IP addresses for a limited period of time. This dynamic-allocation mechanism can reuse the IP address after the lease expires.
An IP address is assigned as follows:
- The client sends a DHCPDISCOVER message to the local network using a 255.255.255.255 broadcast.
- BOOTP relay agents (routers) can forward the DHCPDISCOVER message to the DHCP server in another subnet.
DNS
DNS servers return destination IP addresses given a domain name. DNS is a distributed database. Separate, independent organizations administer their assigned domain name spaces and can break their domains into a number of subdomains. For example, given www.cisco.com, DNS returns the IP address 198.133.219.25. DNS was first specified by RFCs 882 and 883. The current specifications are specified in RFCs 1034 and 1035.
DNS was implemented to overcome the limitations of managing a single text-host table. Imagine creating and maintaining text files with the names and IP addresses of all the hosts in the Internet! DNS scales hostname-to-IP-address translation by distributing responsibility for the domain name space. DNS follows a reversed tree structure for domain name space,
DNS uses TCP and UDP Port 53. UDP is the recommended transport protocol for DNS queries. TCP is the recommended protocol for zone transfers between DNS servers. A zone transfer occurs when you place a secondary server in the domain and transfer the DNS information from the primary DNS server to the secondary server. A DNS query searches for the IP address of an FQDN, such as www.cnn.com.
ARP
When a router needs to send an IP packet over an Ethernet network, it needs to find out what 48-bit MAC physical address to send the frame to. Given the destination IP, ARP obtains the destination MAC. The destination MAC can be a local host or the gateway router's MAC address if the destination IP is across the routed network. ARP is described in RFC 826. The local host maintains an ARP table with a list relating IP address to MAC address.
ARP operates by having the sender broadcast an ARP request. Figure 7-7 shows an example of an ARP request and reply. Suppose a router with the IP address 10.1.1.1 has a packet to send to 10.1.1.10 but does not have the destination MAC address in its ARP table. It broadcasts an ARP request to all hosts in a subnet. The ARP request contains the sender's IP and MAC address as well as the target IP address. All nodes in the broadcast domain receive the ARP request and process it. The device with the target IP address sends an ARP reply to the sender with its MAC address information; the ARP reply is a unicast message sent to 10.1.1.1. The sender now has the target MAC address in its ARP cache and sends the frame.
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