strategy is easy to manage for both network administrators and routing protocols. However, this practice wastes address space in some networks. Some subnets have many hosts and some have only a few, but each consumes an entire subnet number Serial lines are the most extreme example because each has only two hosts that can be connected via a serial line subnet. As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. One of the techniques that has resulted is called Variable Length Subnet Masks (VLSM). With VLSM, a network administrator can use a long mask on networks with few hosts and a short mask on subnets with many hosts. However, this technique is more complex than making them all one size and addresses must be assigned carefully Of course in order to use VlSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First(OSPF), Integrated Intermediate System to Intermediate System(Integrated IS-IS), Enhanced Interior Gateway Routing Protocol(Enhanced IGRP), and static routing On some media, such as IEEE 802 LANS, IP addresses are dynamically discovered through the use of two other members of the Internet protocol suite: Address Resolution Protocol(ARP) and Reverse Address Resolution Protocol (RARP). ARP uses broadcast messages to determine the hardware (MAC layer) address corresponding to a particular network-layer address. ARP is sufficiently generic to allow use of IP with virtually any type of underlying media access mechanism. RARP uses broadcast messages to determine the network-layer address associated with a particular hardware address. RARP is especially important to diskless nodes, for which network-layer addresses usually are unknown at boot time 3. 4 Domain Name 3. 4.1 What is a Domain Name? Most computers connected to the Internet are identified by a unique number called an IP address(for instance, 234.208 12 129). IP addresses are neither intuitive( they don't correspond to a geographical location) nor easy to remember(you can prove that by glancing away from this page and then trying to quote the example IP address above Before you can send a letter to someone, you need to know his/her mailing address. If you want to visit a website, you also need to know its 'address, which is the domain name. Here are some examples of domain Amazon. com nternic. net Linux. org Yahoo co uk Natalie- Portman ws Domain name is case insensitive, which means Google. com, google. com and gOOgLE. coM are the same. a dot act as a separator in a domain name. The part after the last dot is called the Top Level Domain(TLD). From the examples above, their TLDs are showed in Table3-1 Table3-1 Domain Name and Top Level Domain (TLD) Domain name Top Level Domain(TLD) com Linux. org org Yahoo co uk Domain name registrations for the most seen com,. net,. org are unrestricted and are opened to anyone. They are usually referred to as International TLDs. When the internet started a decadestrategy is easy to manage for both network administrators and routing protocols. However, this practice wastes address space in some networks. Some subnets have many hosts and some have only a few, but each consumes an entire subnet number. Serial lines are the most extreme example, because each has only two hosts that can be connected via a serial line subnet. As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. One of the techniques that has resulted is called Variable Length Subnet Masks (VLSM). With VLSM, a network administrator can use a long mask on networks with few hosts and a short mask on subnets with many hosts. However, this technique is more complex than making them all one size and addresses must be assigned carefully. Of course in order to use VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated Intermediate System to Intermediate System (Integrated IS-IS), Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), and static routing. On some media, such as IEEE 802 LANs, IP addresses are dynamically discovered through the use of two other members of the Internet protocol suite: Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol (RARP). ARP uses broadcast messages to determine the hardware (MAC layer) address corresponding to a particular network-layer address. ARP is sufficiently generic to allow use of IP with virtually any type of underlying media access mechanism. RARP uses broadcast messages to determine the network-layer address associated with a particular hardware address. RARP is especially important to diskless nodes, for which network-layer addresses usually are unknown at boot time. 3.4 Domain Name 3.4.1 What is a Domain Name? Most computers connected to the Internet are identified by a unique number called an IP address (for instance, 234.208.12.129). IP addresses are neither intuitive (they don't correspond to a geographical location) nor easy to remember (you can prove that by glancing away from this page and then trying to quote the example IP address above! Before you can send a letter to someone, you need to know his/her mailing address. If you want to visit a website, you also need to know its 'address', which is the domain name. Here are some examples of domain names: ▪ Google.com ▪ Amazon.com ▪ Internic.net ▪ Linux.org ▪ Yahoo.co.uk ▪ Natalie-Portman.ws Domain name is case insensitive, which means Google.com, google.com and gOOgLE.coM are the same. A dot act as a separator in a domain name. The part after the last dot is called the Top Level Domain (TLD). From the examples above, their TLDs are showed in Table3-1. Table3-1 Domain Name and Top Level Domain (TLD) Domain Name Top Level Domain (TLD) Google.com .com Amazon.com .com Internic.net .net Linux.org .org Yahoo.co.uk .uk Natalie-Portman.ws .ws Domain name registrations for the most seen .com, .net, .org are unrestricted and are opened to anyone. They are usually referred to as International TLDs. When the internet started a decade