CCNA v3.0 Semester 3 2 What is VLSM? A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule. VLSM allows an organization to use more than one subnet mask within the same network address space. It is often referred to as ‘subnetting a subnet’, and can be used to maximize addressing efficiency. Large subnets are created for addressing LANs and small subnets are created for WAN links (a 30 bit mask is used to create subnets with only two host).
CCNA v3.0 Semester 3 3 Subnetting vs. VLSM Subnetting allows you to divide big networks into smaller, equal-sized slices. VLSM allows you to divide big networks into smaller, different-sized slices. This enables you to make maximum use of your valuable IP address space. So basically, you are now utilizing subnet masks in the same IP address space.
CCNA v3.0 Semester 3 5 Addressing a Network with Standard Subnetting Site A has two Ethernet networks Site B had one Ethernet network Site C had one Ethernet network /24 How many network addresses are needed? How many hosts are needed for the largest LAN? How many bits need to be borrowed to address this network? Site A Site BSite C 25 users 10 users8 users
CCNA v3.0 Semester 3 6 Addressing a Network with Standard Subnetting Site A has two Ethernet networks Site B had one Ethernet network Site C had one Ethernet network Site A Site BSite C 25 users 10 users8 users If we borrow 3 bits from a class C address, that will give us eight networks, but we can only use six of them. Each network will have 30 usable addresses. It will take four network addresses to accommodate the Ethernet networks at each site. That leaves us with two extra networks. There is also a point-to-point WAN connection between each site. These two connections will take up the remaining two networks.
CCNA v3.0 Semester 3 7 Addressing a Network with Standard Subnetting Borrowing 3 bits will meet the current needs of the company, but it leaves little room for growth. Each network will have 30 usable addresses, including the point-to-point WAN links (which only require two addresses). Site A Site BSite C 25 users 10 users8 users
CCNA v3.0 Semester 3 8 We can use subnet 0 To enable subnet 0 on a Cisco router (if not already enabled), it is necessary to use the global configuration command ip subnet-zero. Router# configure terminal (config t) Router(config)# ip subnet-zero To disable subnet 0, use the no form of this command. Router(config)# no ip subnet-zero
CCNA v3.0 Semester 3 9 Subnetting in a Box In a class C network there are 256 addresses. Provides 1 network with 256 addresses. When we subnet the address, we break it down in to smaller units or subnets. Subnet mask: addresses
CCNA v3.0 Semester 3 10 Subnetting in a Box Borrowing 1 bit would break the 256 addresses in to two parts (networks) Providing 2 networks each with 128 addresses. Subnet mask: addresses
CCNA v3.0 Semester 3 11 Subnetting in a Box Borrowing 2 bits would break each of the 2 networks in half again. Providing 4 networks, each with 64 addresses. Subnet mask: addresses
CCNA v3.0 Semester 3 12 Subnetting in a Box Borrowing 3 bits would break each of these 4 networks in half again. Providing 8 networks, each with 32 addresses. Subnet mask: addresses addresses addresses addresses
CCNA v3.0 Semester 3 13 Subnetting in a Box Borrowing 4 bits would break each of these 8 networks in half again. Providing 16 networks, each with 16 addresses. Subnet mask: addresses
CCNA v3.0 Semester 3 14 Addressing a Network Using VLSM When using VLSM to subnet an address, not all of the subnets have to be the same size. A different subnet mask may be applied to some of the subnets to further subnet the address. In order to take advantage of VLSM, the proper routing protocol must be selected. Not all routing protocols share subnetting information in their routing table updates.
CCNA v3.0 Semester 3 15 Addressing a Network Using VLSM To subnet using VLSM, identify the LAN with the largest number of hosts. Subnet the address /24 based on this information. Site A has two Ethernet networks (25 hosts each) Site B had one Ethernet network (10 hosts) Site C had one Ethernet network (8 hosts) Site A Site BSite C 25 users 10 users8 users
CCNA v3.0 Semester 3 16 Addressing a Network Using VLSM Subnet 1 & 2 to address Site A Ethernet networks. Subnet 5 to accommodate Site B & C Ethernet networks. Subnet 6 can be subnetted to accommodate the WAN links. Site A Site BSite C 25 users 10 users8 users Free Addresses Site A Site B & C Site B Site C WAN links Free Addresses WAN 1 & 2
CCNA v3.0 Semester 3 17 Addressing a Network Using VLSM Through applying VLSM, the topology was able to be addressed and still have two complete subnets available for future growth. Site A Site BSite C 25 users 10 users8 users / / / / / /30
CCNA v3.0 Semester 3 18 Addressing a Network Using VLSM Exercise 1 Your company IP network is /24. Headquarters is connected to five branch offices by a WAN link, and to an ISP. Determine an appropriate IP addressing scheme. (the ISP owns the addresses on its link) Headquarters Branch 1 60 users 12 users Branch 2Branch 3Branch 4Branch 5 ISP
CCNA v3.0 Semester /24 Subnet according to the largest subnet needed. (Headquarters 60 hosts) Borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.
CCNA v3.0 Semester 3 20 Playing it safe, we will not use the first subnet (subnet 0) We will start addressing with /26. Headquarters needs 60 hosts, so we will assign them Headquarters 60 hosts 26 bit mask or /26 ( )
CCNA v3.0 Semester 3 21 The 5 Branch offices need 12 hosts each The next address block available is the block. Use VLSM. Headquarters 60 hosts 26 bit mask or /26 ( ) Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices Branch 1 12 hosts /28 ( ) Branch 2 12 hosts /28 ( ) Branch 3 12 hosts /28 ( ) Branch 4 12 hosts /28 ( )
CCNA v3.0 Semester 3 22 To obtain a block for Branch 5, we will need to subnet the block using a /28 mask Headquarters 60 hosts 26 bit mask or /26 ( ) Branch 1 12 hosts /28 ( ) Branch 2 12 hosts /28 ( ) Branch 3 12 hosts /28 ( ) Branch 4 12 hosts /28 ( ) Branch 5 12 hosts /28 ( )
CCNA v3.0 Semester 3 23 Now connect the 5 WAN links to the Branch offices. These are point- to-point connections and only require 2 addresses Here we will use a /30 mask to further subnet the subnets. Headquarters 60 hosts 26 bit mask or /26 ( ) Branch 1 12 hosts /28 ( ) Branch 2 12 hosts /28 ( ) Branch 3 12 hosts /28 ( ) Branch 4 12 hosts /28 ( ) Branch 5 12 hosts /28 ( ) WAN 1 WAN 2 WAN 3 WAN WAN 5 240
CCNA v3.0 Semester 3 24 Any remaining networks could be used for future growth of either LANs or WANs. Subnet 0 could also be further subnetted according to the needs of the network Headquarters 60 hosts 26 bit mask or /26 ( ) Branch 1 12 hosts /28 ( ) Branch 2 12 hosts /28 ( ) Branch 3 12 hosts /28 ( ) Branch 4 12 hosts /28 ( ) Branch 5 12 hosts /28 ( ) WAN 1 WAN 2 WAN 3 WAN WAN 5 240
CCNA v3.0 Semester 3 25 Address provided by ISP / / / / / / / / / / /30 Applying the Addresses to the Topology
CCNA v3.0 Semester 3 26 Classful Addressing The IPv4 address architecture uses (a/n) 8 bit network number for Class A addresses 16 bit network number for Class B addresses 24 bit network number for Class C addresses Class B Network Host 10 Class C Network Host 110 Class A Network Host 0
CCNA v3.0 Semester 3 28 Classless Interdomain Routing CIDR (pronounced “cider”) ignores class. Using CIDR, a router views a bit mask to determine the network and host portions of an address. This allows CIDR to craft network address spaces according to the size of a network instead of force- fitting networks into pre-sized network address spaces.
CCNA v3.0 Semester 3 29 Classless Interdomain Routing CIDR sounds a lot like VLSM CIDR is usually discussed in general Internet context (ISPs) –Uses custom length prefixes to reduce workload in key Internet routers VLSM is usually discussed in enterprise context –Uses custom length prefixes to have better usage of enterprise address space
CCNA v3.0 Semester 3 30 Classless Interdomain Routing Routers use the network-prefix, rather than the first 3 bits of the IP address, to determine the dividing point between the network number and the host number. In the CIDR model, each piece of routing information is advertised with a bit mask or prefix-length ( /x ). The prefix-length is a way of specifying the number bits in the network-portion of each routing table entry.
CCNA v3.0 Semester 3 31 Classless Interdomain Routing For example, a network with 20 bits of network-number and 12 bits of host-number would be advertised with a 20 bit prefix (/20). The clever thing is that the IP address advertised with the /20 prefix could be a former Class A, Class B, or Class C. All addresses with a /20 prefix represent the same amount of address space (2 12 or 4,096 host addresses). 20 bits network + 12 bits host
CCNA v3.0 Semester 3 32 Classless Interdomain Routing Address space can now be assigned in “chunks” that fit the need. If an organization needs 254 host addresses, what difference does it make whether they are given: – a Class C ( /24) – 1 / 256th of a Class B ( /24) – 1 / 65,536th of a Class A ( /24) Using a /24 prefix, each of these specifies eight host bits which will support 254 hosts.
CCNA v3.0 Semester 3 33
CCNA v3.0 Semester 3 34 Route Aggregation w/ CIDR or (Summarization) You need 500 addresses. Given two consecutive /24 addresses: ( /24 and /24) This address space could be advertised to the rest of the Internet as /23. Why? (the two /24s have the first 23 bits in common) bits network prefix
CCNA v3.0 Semester 3 35 CIDR Scenario continued If the ISP owns all of the networks (256 /24s), why should it advertise all of them separately? Instead, it could simply advertise /16 (which would be /24 through /24). This would reduce the size of the routing tables on the router to which the routes are advertised bits network prefix
CCNA v3.0 Semester 3 36 CIDR Scenario continued The summary of route /23 is called a “CIDR block” or a supernet. Because we are dealing with binary, the block size is always a power of two (2, 4, 8, 16, 32, etc.). The starting point of the block must be a multiple of the power of two that is being used (2 1 … 2, 4, 6, 8, etc.). – – – – – Examples of starting addresses
CCNA v3.0 Semester 3 40 CIDR in a Nutshell Hand out pieces of classful networks (to avoid wasting addresses) Identify the network portion of an address with a network prefix ( /x) Advertise blocks of networks (to reduce the size of routing tables).
CCNA v3.0 Semester 3 41 CIDR Example Objective Create an addressing scheme using VLSM. Scenario You are assigned the CIDR address /22 and you must support the network shown in the diagram. Create an addressing scheme that will meet the diagram requirements. 300 users 100 users
CCNA v3.0 Semester 3 42 Dissect the problem Given the CIDR address /22 How many /24 networks do we have? How many host addresses do we have? What is the largest LAN requirement? 300 users 100 users
CCNA v3.0 Semester 3 43 Address given /22 Host required - 300, 100, 100, 100, and 3 WAN links
CCNA v3.0 Semester 3 44 Address given /22 Host required - 300, 100, 100, 100, and 3 WAN links hosts /
CCNA v3.0 Semester 3 45 Address given /22 Host required - 300, 100, 100, 100, and 3 WAN links hosts / hosts / hosts /
CCNA v3.0 Semester 3 46 Address given /22 Host required - 300, 100, 100, 100, and 3 WAN links hosts / hosts / hosts / hosts /
CCNA v3.0 Semester 3 47 Address given /22 Host required - 300, 100, 100, 100, and 3 WAN links hosts / hosts / hosts / hosts / WAN links /
CCNA v3.0 Semester 3 48 CIDR Result Given the CIDR address / users 100 users / / / / / / /30 Two /24s
CCNA v3.0 Semester 3 49 Classless Interdomain Routing For the router to operate in a classless manner and match destination IP addresses to a CIDR network address, The global command: ip classless must be configured. Router(config)# ip classless
CCNA v3.0 Semester 3 50 Routing Information Protocol (RIP) RIP is a relatively old, but still commonly used interior gateway protocol (IGP). It was created for use in small homogeneous networks. It is a distance-vector protocol that is used with classful IP addressing only. RIP v1 sends routing update messages at regular intervals (30 seconds) and when the network topology changes. RIP uses hop count as its only metric and maintains only the best route to a destination.
CCNA v3.0 Semester 3 51 RIP Version 2 Known as RIP V2 In RIP v2 all of the operation procedures, timers, and stability functions of RIP v1 remain the same in version 2, with the exception of the broadcast updates. RIP v2 has become the standard version of RIP used in networks today.
CCNA v3.0 Semester 3 52 RIP V2 is RIP V1 with extensions Subnet masks carried with each route entry Authentication of routing updates Next-hop addresses carried with each route entry External route tags Multicast route updates
CCNA v3.0 Semester 3 53 RIP v2 The most important of these extensions is the addition of a Subnet Mask field This enables the use of variable-length subnet masks (VLSMs) and qualifies RIP v2 as a classless routing protocol. RIP v2 Packet Format RIP v1 Packet Format
CCNA v3.0 Semester 3 54 RIP v2 RIP v2 allocated a 4-octet field to associate a subnet mask to a destination IP address. When used in tandem, the IP address and its subnet mask enable RIP v2 to specifically identify the type of destination that the route leads to. This allows RIP v2 to route specific subnets, regardless of whether the subnet mask is fixed or of variable length. RIP v2 Packet Format
CCNA v3.0 Semester 3 55 RIP v2 RIP v2 differs from RIP v1 in the way update are sent out. RIP v1 sends updates as a broadcast (all stations receive the broadcast message) RIP v1 does not send subnet mask information in its updates. RIP v2 sends updates as a multi-cast. Multi-casting is a technique for simultaneously advertising routing information to multiple RIP devices via the class D address
CCNA v3.0 Semester 3 56 RIP v1 & RIP v2 comparisons Both use hop count as a metric Both have the same metric value for infinite distance (16) Both use split horizon to prevent routing loops. RIP v1 broadcasts routing table updates, while RIP v2 multicasts its updates
CCNA v3.0 Semester 3 57 Configuring RIP v1 To configure RIP v1 on a router, enter the following commands: Router# config t Router(config)# router rip Router(config-router)# network NOTE - If no version is specified in the configuration, version 1 will be used. The router will listen for version 1 and 2 updates but send only version 1.
CCNA v3.0 Semester 3 58 Configuring RIP v2 To take advantage of version 2s features, it is necessary to turn off version 1 support and enable version 2 updates with the following commands: Router(config)# router rip Router(config-router)# version 2 Router(config-router)# network NOTE - The default behavior can be restored by entering the command no version in the config-router mode. Router(config)# router rip Router(config-router)# no version
CCNA v3.0 Semester 3 59 Verifying & Troubleshooting RIP show ip route to make sure routers have learned all networks dynamically show ip protocols to see information about the routing protocols used. debug ip RIP to see live routing updates
CCNA v3.0 Semester 3 60 You can override the default behavior of RIP by configuring a particular interface to behave differently. Overriding Default Behavior of RIP Interface e0 sends and receives version 1 updates only. RIP v2 configured on the router. Router(config)# router rip Router(config-router)# version 2 Router(config-router)# network Router(config-router)# exit Router(config)# int e0 Router(config-if)# ip address Router(config-if)# ip rip send version 1 Router(config-if)# ip rip receive version 1
CCNA v3.0 Semester 3 61 You can override the default behavior of RIP by configuring a particular interface to behave differently. Interface e2 has no special configuration and therefore sends and receives version 2 by default. Overriding Default Behavior of RIP Interface e1 sends and receives both version 1 and 2 updates. Router(config)# int e1 Router(config-if)# ip address Router(config-if)# ip rip send version 1 2 Router(config-if)# ip rip receive version 1 2 Router(config)# int e2 Router(config-if)# ip address
CCNA v3.0 Semester 3 64 Configuring static routes w/ next- hop IP address Administrative distance of 1 - default Next hop interface
CCNA v3.0 Semester 3 65 Configuring Static Routes Remember, an administrator actually enters these routes into the routing table. That makes them static route entries – because the router is not “discovering” those routes. If for some reason that outgoing interface goes down or is not available for some reason, then at that time the route will be removed from the routing table. Show ip route shows the routing table. The route would still be in the configuration (because it was entered globally), but that route could now no longer be used by the router because the interface it refers to is down for some reason.
CCNA v3.0 Semester 3 66 Administrative Distance What is the default for a outgoing interface? What is the default for the next-hop address? Defaults can always be changed!!! Just make it higher if you want it to be a “backup” route. ip route
CCNA v3.0 Semester 3 67 Router A Router B Router C S /24 S /24 S /24 S / / / /24 What would you enter to configure a static route from Router C to the LAN on Router A using outgoing interface? The LAN on Router B from Router A using next-hop?
CCNA v3.0 Semester 3 68 The static default route A router should be configured with a special type of static route – a default route. This default route routes packets with destinations that do not match any of the other routes in the routing table It is a “gateway of last resort” that allows the router to forward “destination unknown” packets out a particular interface ip route [next-hop- address | outgoing interface]