CCNA :) Be a Good Network Administrator

1.2  RIP Version 2 1.2.1 RIP history This page will explain the functions and limitations of RIP. The Internet is a collection of autonomous systems (AS). Each AS is generally administered by a single entity. Each AS has a routing technology which can differ from other autonomous systems. The routing protocol used within an AS is referred to as an Interior Gateway Protocol (IGP). A separate protocol used to transfer routing information between autonomous systems is referred to as an Exterior Gateway Protocol (EGP). RIP is designed to work as an IGP in a moderate-sized AS. It is not intended for use in more complex environments. RIP v1 is considered a classful IGP. RIP v1 is a distance vector protocol that broadcasts the entire routing table to each neighbor router at predetermined intervals. The default interval is 30 seconds. RIP uses hop count as a metric, with 15 as the maximum number of hops. If the router receives information about a network, and the receiving interface belong...

  Configuring VLSM 1.1.6 This page will teach students how to calculate and configure VLSM. If VLSM is the scheme chosen, it must then be calculated and configured correctly. The following are VLSM calculations for the LAN connections in Figure : Network address: 192.168.10.0 The Perth router has to support 60 hosts. That means a minimum of six bits are needed in the host portion of the address. Six bits will yield 2 6 – 2, or 62 possible host addresses. The LAN connection for the Perth router is assigned the 192.168.10.0/26 subnet. The Sydney and Singapore routers have to support 12 hosts each. That means a minimum of four bits are needed in the host portion of the address. Four bits will yield 2 4 – 2, or 14 possible host addresses. The LAN connection for the Sydney router is assigned the 192.168.10.96/28 subnet and the LAN connection for the Singapore router is assigned the 192.168.10.112/28 subnet. The KL ro...

  Route aggregation with VLSM 1.1.5  When VLSM is used, it is important to keep the subnetwork numbers grouped together in the network to allow for aggregation. For example, networks like 172.16.14.0 and 172.16.15.0 should be near one another so that the routers only carry a route for 172.16.14.0/23. The use of classless interdomain routing (CIDR) and VLSM prevents address waste and promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997. Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points with a prefix address, until the entire network is advertised as a single aggregate route of 200.199.48.0/20. Route summarization, or supernetting, is only possible if the routers of a network use a classless routing protocol, such as OSPF or EIGRP. Classless routing proto...

 Calculating subnets with VLSM 1.1.4 VLSM helps to manage IP addresses. This page will explain how to use VLSM to set subnet masks that fit the link or segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another. The example in Figure shows a network that requires an address scheme. The example contains a Class B address of 172.16.0.0 and two LANs that require at least 250 hosts each. If the routers use a classful routing protocol, the WAN link must be a subnet of the same Class B network. Classful routing protocols such as RIP v1, IGRP, and EGP do not support VLSM. Without VLSM, the WAN link would need the same subnet mask as the LAN segments. A 24-bit mask of 255.255.255.0 can support 250 hosts.   The WAN link only needs two addresses, one for each router. That means that 252 addresses would be wasted. If VLSM was used, a 24-bit mask would still be applied on the LAN segments for ...

When to Use VLSM 1.1.3  It is important to design an address scheme that allows for growth and does not waste addresses. This page examines how VLSM can be used to prevent the waste of addresses on point-to-point links. As shown in Figure , the network management team has decided to avoid the wasteful use of the /27 mask on the point-to-point links. The team applies VLSM to the address problem. To apply VLSM to the address problem, the team breaks the Class C address into subnets of variable sizes. Large subnets are created for LANs. Very small subnets are created for WAN links and other special cases. A 30-bit mask is used to create subnets with only two valid host addresses. This is the best solution for the point-to-point connections. The team will take one of the three subnets they previously decided to assign to the WAN links, and subnet it again with a 30-bit mask. In the example, the team has taken one of the last three subnets, subnet 6, and subnetted it again. This time th...

A Waste of VLSM 1.1.2  This page will explain how certain address schemes can waste address space. In the past, the first and last subnet were not supposed to be used. The use of the first subnet, which was known as subnet zero, was discouraged because of the confusion that could occur if a network and a subnet had the same address. This also applied to the use of the last subnet, which was known as the all-ones subnet. With the evolution of network technologies and IP address depletion, the use of the first and last subnets have become an acceptable practice in conjunction with VLSM. In Figure , the network management team has borrowed three bits from the host portion of the Class C address that has been selected for this address scheme. If the team decides to use subnet zero, there will be eight useable subnets. Each subnet can support 30 hosts. If the team decides to use the no ip subnet-zero command, there will be seven usable subnets with 30 hosts in each subnet. Cisco ...

  VLSM 1.1.1   What is VLSM and why is it used? As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. This page introduces a technique called 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. - In order to implement VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated IS-IS, Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2, and static routing. VLSM allows an organization to use more than one subnet mask within the same network address space. VLSM implementation maximizes address efficiency, and is often referred to as subnetting a subnet. Classful routing protocols require that a single network use the same subnet mask. As an example, a network with an address of 192.168.187.0 can use just one subnet mask, such as 255.255.25...