Friday, April 30, 2021

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 the team uses a 30-bit mask. Figures and illustrate that after using VLSM, the team has eight ranges of addresses to be used for the point-to-point links.

The next page will teach students how to calculate subnets with VLSM.




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 the 250 hosts. A 30-bit mask could be used for the WAN link because only two host addresses are needed.

Figure shows where the subnet addresses can be applied based on the number of host requirements. The WAN links use subnet addresses with a prefix of /30. This prefix allows for only two host addresses which is just enough for a point-to-point connection between a pair of routers.

In Figure , the subnet addresses used are generated when the 172.16.32.0/20 subnet is divided into /26 subnets.

To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26 subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of /30. This provides four more subnet bits and therefore 16 (24) subnets for the WANs. Figure illustrates how to work through a VLSM system.

VLSM can be used to subnet an already subnetted address. For example, consider the subnet address 172.16.32.0/20 and a network that needs ten host addresses. With this subnet address, there are 212 – 2, or 4094 host addresses, most of which will be wasted. With VLSM it is possible to subnet 172.16.32.0/20 to create more network addresses with fewer hosts per network. When 172.16.32.0/20 is subnetted to 172.16.32.0/26, there is a gain of 26, or 64 subnets. Each subnet can support 26 – 2, or 62 hosts.

Use the following steps to apply VLSM to 172.16.32.0/20:

  1. Write 172.16.32.0 in binary form.
  2. Draw a vertical line between the 20th and 21st bits, as shown in Figure . The original subnet boundary was /20.
  3. Draw a vertical line between the 26th and 27th bits, as shown in Figure . The original /20 subnet boundary is extended six bits to the right, which becomes /26.
  4. Calculate the 64 subnet addresses with the bits between the two vertical lines, from lowest to highest in value. The figure shows the first five subnets available.

It is important to remember that only unused subnets can be further subnetted. If any address from a subnet is used, that subnet cannot be further subnetted. In Figure , four subnet numbers are used on the LANs. The unused 172.16.33.0/26 subnet is further subnetted for use on the WAN links.

The Lab Activity will help students calculate VLSM subnets.

The next page will describe route aggregation.


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 routers with Cisco IOS version 12.0 or later, use subnet zero by default.

In Figure , the Sydney, Brisbane, Perth, and Melbourne remote offices may each have 30 hosts. The team realizes that it has to address the three point-to-point WAN links between Sydney, Brisbane, Perth, and Melbourne. If the team uses the last three subnets for the WAN links, all of the available addresses will be used and there will be no room for growth. The team will also have wasted the 28 host addresses from each subnet to simply address three point-to-point networks. This address scheme would waste one-third of the potential address space.

Such an address scheme is fine for a small LAN. However, it is extremely wasteful if point-to-point connections are used.

The next page will explain how VLSM can be used to prevent wasted addresses.

 


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 255.255.255.0.

A routing protocol that allows VLSM gives the network administrator freedom to use different subnet masks for networks within a single autonomous system.  Figure shows an example of how a network administrator can use a 30-bit mask for network connections, a 24-bit mask for user networks, and even a 22-bit mask for networks with up to 1000 users.

The next page will discuss network address schemes.


Semester 3 

Module 1: Introduction to Classless Routing - Overview

Network administrators must anticipate and manage the physical growth of networks. This may require them to buy or lease another floor of a building for new network equipment such as racks, patch panels, switches, and routers. Network designers must choose address schemes that allow for growth. Variable-length subnet mask (VLSM) is used to create efficient and scalable address schemes.

Almost every enterprise must implement an IP address scheme. Many organizations select TCP/IP as the only routed protocol to run on their networks. Unfortunately, the architects of TCP/IP did not predict that the protocol would eventually sustain a global network of information, commerce, and entertainment.

IPv4 offered an address strategy that was scalable for a time before it resulted in an inefficient allocation of addresses. IPv4 may soon be replaced with IP version 6 (IPv6) as the dominant protocol of the Internet. IPv6 has virtually unlimited address space and implementation has begun in some networks. Over the past two decades, engineers have successfully modified IPv4 so that it can survive the exponential growth of the Internet. VLSM is one of the modifications that has helped to bridge the gap between IPv4 and IPv6.

Networks must be scalable since the needs of users evolve. When a network is scalable it is able to grow in a logical, efficient, and cost-effective way. The routing protocol used in a network helps determine the scalability of the network. It is important to choose the routing protocol wisely. Routing Information Protocol version 1 (RIP v1) is suitable for small networks. However, it is not scalable to large networks. RIP version 2 (RIP v2) was developed to overcome these limitations.

This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.  

Students who complete this module should be able to perform the following tasks:

  • Define VLSM and briefly describe the reasons for its use
  • Divide a major network into subnets of different sizes using VLSM
  • Define route aggregation and summarization as they relate to VLSM
  • Configure a router using VLSM
  • Identify the key features of RIP v1 and RIP v2
  • Identify the important differences between RIP v1 and RIP v2
  • Configure RIP v2
  • Verify and troubleshoot RIP v2 operation

Configure default routes using the ip route and ip default-network commands 


Saturday, April 18, 2020

WAN Design

2.3 WAN Design 
2.3.1 WAN Communication
WANS are considered to be a set of data links connecting routers on LANs. User end stations and servers on LANs exchange data. Routers pass data between networks across the data links.
Because of cost and legal reasons, a communications provider or a common carrier normally owns the data links that make up a WAN. The links are made available to subscribers for a fee and are used to interconnect LANs or connect to remote networks. WAN data transfer speed (bandwidth) is considerably slower than the 100 Mbps that is common on a LAN. The charges for link provision are the major cost element of a WAN and the design must aim to provide maximum bandwidth at acceptable cost. With user pressure to provide more service access at higher speeds and management pressure to contain cost, determining the optimal WAN configuration is not an easy task.
WANs carry a variety of traffic types such as data, voice, and video. The design selected must provide adequate capacity and transit times to meet the requirements of the enterprise. Among other specifications, the design must consider the topology of the connections between the various sites, the nature of those connections, and bandwidth capacity.
Older WANs often consisted of data links directly connecting remote mainframe computers. Today’s WANs, though, connect geographically separated LANs. End-user stations, servers, and routers communicate across LANs, and the WAN data links terminate at local routers. By exchanging Layer 3 address information about directly connected LANs, routers determine the most appropriate path through the network for the required data streams. Routers can also provide quality of service (QoS) management, which allots priorities to the different traffic streams.
Because the WAN is merely a set of interconnections between LAN based routers, there are no services on the WAN. WAN technologies function at the lower three layers of the OSI reference model. Routers determine the destination of the data from the network layer headers and transfer the packets to the appropriate data link connection for delivery on the physical connection.

Wednesday, May 1, 2019

Cable Modem

2.2.8 Cable Modem
Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from some cable television networks. This allows for greater bandwidth than the conventional telephone local loop.
Enhanced cable modems enable two-way, high-speed data transmissions using the same coaxial lines that transmit cable television. Some cable service providers are promising data speeds up to 6.5 times that of T1 leased lines. This speed makes cable an attractive medium for transferring large amounts of digital information quickly, including video clips, audio files, and large amounts of data. Information that would take two minutes to download using ISDN BRI can be downloaded in two seconds through a cable modem connection.
Cable modems provide an always-on connection and a simple installation. An always-on cable connection means that connected computers are vulnerable to a security breach at all times and need to be suitably secured with firewalls. To address security concerns, cable modem services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site.
A cable modem is capable of delivering up to 30 to 40 Mbps of data on one 6 MHz cable channel. This is almost 500 times faster than a 56 Kbps modem.
With a cable modem, a subscriber can continue to receive cable television service while simultaneously receiving data to a personal computer. This is accomplished with the help of a simple one-to-two splitter.
Cable modem subscribers must use the ISP associated with the service provider. All the local subscribers share the same cable bandwidth. As more users join the service, available bandwidth may be below the expected rate.

DSL

2.2.7 DSL
Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typical LANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The term xDSL covers a number of similar yet competing forms of DSL technologies:
  • Asymmetric DSL (ADSL)
  • Symmetric DSL (SDSL)
  • High Bit Rate DSL (HDSL)
  • ISDN (like) DSL (IDSL)
  • Consumer DSL (CDSL), also called DSL-lite or G.lite
DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop copper lines. DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DSL Access Multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a less cumbersome single medium, generally a T3/DS3 connection. Current DSL technologies are using sophisticated coding and modulation techniques to achieve data rates up to 8.192 Mbps.
The voice channel of a standard consumer telephone covers the frequency range of 330 Hz to 3.3 KHz. A frequency range, or window, of 4 KHz is regarded as the requirements for any voice transmission on the local loop. DSL technologies place upload (upstream) and download (downstream) data transmissions at frequencies above this 4 KHz window. This technique is what allows both voice and data transmissions to occur simultaneously on a DSL service.
The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service are categorized as ADSL or SDSL and there are several varieties of each type. Asymmetric service provides higher download or downstream bandwidth to the user than upload bandwidth. Symmetric service provides the same capacity in both directions.
Not all DSL technologies allow the use of a telephone. SDSL is called dry copper because it does not have a ring tone and does not offer telephone service on the same line. Therefore a separate line is required for the SDSL service.
The different varieties of DSL provide different bandwidths, with capabilities exceeding those of a T1 or E1 leased line. The transfer rates are dependent on the actual length of the local loop and the type and condition of its cabling. For satisfactory service, the loop must be less than 5.5 kilometers (3.5 miles). DSL availability is far from universal, and there are a wide variety of types, standards, and emerging standards. It is not a popular choice for enterprise computer departments to support home workers. Generally, a subscriber cannot choose to connect to the enterprise network directly, but must first connect to an Internet service provider (ISP). From here, an IP connection is made through the Internet to the enterprise. Thus, security risks are incurred. To address security concerns, DSL services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site.