from subnetting to vlsm classful vs. classless routing vlsm explained why vlsm suggestions for...
TRANSCRIPT
From Subnettingto VLSM
From Subnettingto VLSM
Classful vs. Classless Routing
VLSM Explained
Why VLSM
Suggestions for Teaching VLSM
Credits
• Virginia Phillips, CCNA, CCAI– Instructor CCNP classes, Youngstown State
University
• Edmund Ickert, CCNA, CCAI– Instructor CCNA classes, Youngstown State
University, completed all CCNP courses
• Sandeep Kolwalkar, CCNA– Graduate Student, taking CCNP classes,
Youngstown State University
Classful vs Classless Routing
• Classful routing assigns address space based on the value in the first octet of the 32-bit IP address– RFC Number 791 (760)– Class based on value in first octet value– Receiving router ands subnet mask to determine subnet
• Class A 0-126• Class B 128-191• Class C 192-223
• Classless routing ignores classes and uses a CIDR value (number of 1s in network mask) to identify the network– CIDR transmitted as part of IP address – RFC 1517-1520– Network portion not restricted to entire octet
Classless RoutingAddress Space Issues
• Class A and Class B = 75% address space– < 17000 organizations can be assigned address
• Class C = 12.5% available address space– Each network limited to 254 maximum hosts– Potential routing problems
• Too many network addresses in routing table• Extra work for CPU; more memory required
Private AddressingRFC 1918
• Class A 10.0.0.0 to 10.255.255.255
• Class B 172.16.0.0 to 172.31.255.255
• Class C 192.168.0.0 to 192.168.255.255– Used to extend life of IPv4 addressing– Note: Do not mix private and public IP address in
same network – it will create discontiguous subnets which causes problems
Classless Routing
• Another method used to extend the life of IPv4• Temporary solution to deal with lack of network
numbers • Uses bit mask (NOT 1st octet value) to determine
network portion of address• Uses CIDR to summarize routing information;
CIDR transmitted with IP address• Enables the use of supernets and/or route
aggregation and summarization– Smaller routing tables– Reduced router memory requirements– Reduced number of CPU cycles for routing processes
Routing Protocols
• Classful – can’t send subnet information in updates– RipV1, IGRP, EGP, BGP3 – also can’t support discontiguous
subnets
• Classless – Sends CIDR in updates sent via multicasting– Can authenticate
• RipV2 (RFC 1058), EIGRP, OSPF, IS-IS, BGP4– RIPV2 and EIGRP automatically summarize at classful boundary
unless you configure differently» RouterA (config-router) no auto-summary
VLSMVariable Length Subnet Masking
• Subnets a subnet
• Can support multiple contiguous routes
• Can use more than one subnet mask for address space allocated to a firm
• Makes more efficient use of available address space– Creates two-host subnets for serial links
Why Not IPv6?128-bit address space
• Slow to arrive
• IPv4 revitalized with new features– VLSM, NAT/PAT, IP unnumbered, private
addresses
• Not supported by legacy systems
• Requires new software (and hardware)
• Requires retraining
Zero Subnet (Ones too?)
• Zero subnet– IOS 12.X and higher supports by default– Configure pre-12.x IOS routers
• RouterA(config) IP subnet-zero
– DO Use it to increase address space available
• Ones subnet– Defined in RFC 1878– Can use it; however can cause problems – Avoid using unless you absolutely need it
Route Aggregation Example 1
• Assume you are using three Class B private addresses– 172.16.0.0 10101100.000100 00.0.0– 172.17.0.0 10101100.000100 01.0.0– 172.18.0.0 10101100.000100 10.0.0
• Common bits are 10111000.0001– 8 bits in first octet + 6 bits in second octet = 14– CIDR is 14
• Insulates upstream routers from route flapping problems (serial link problem)
Route Aggregation Example 2
• Assume you are using three Class A private addresses– 10.20.0.0 00001010.000101 00.0.0– 10.21.0.0 00001010.000101 01.0.0– 10.22.0.0 00001010.000101 10.0.0
• Common bits are 00001010.000101– 8 bits in first octet + 6 bits in second octet = 14– CIDR is 14
Supernet Example 1
• Company assigned 4 contiguous Class C networks– 200.10.10.0 11001000.00001010.00001010.0– 200.10.11.0 11001000.00001010.00001011.0– 200.10.12.0 11001000.00001010.00001100.0– 200.10.13.0 11001000.00001010.00001101.0
• Summarize on common bits = 21
• Appears in routing table as 200.10.10.0/21
Supernet Example 2
• Company assigned 4 contiguous Class C networks– 200.10.101.0 11001000.00001010.11001001.0– 200.10.102.0 11001000.00001010.11001010.0– 200.10.103.0 11001000.00001010.11001011.0– 200.10.104.0 11001000.00001010.11001100.0
• Summarize on common bits = 21
• Appears in routing table as 200.10.101.0/21
Network Subnet Example
• 128.1.0.0/16 is assigned IP address– 130 subnets needed – Requires use of third octet for subnet values
• 1,2,3,4, …., 254
– Each subnet can support 254 hosts– Each serial connection will use a subnet and waste
252 address spaces
Network Subnet Example
• Assigned IP address is 128.1.0.0– Scenario - 130 subnets needed and 20 serial
connections used now– Requires use of third octet for subnets
• 128.1.0.0 to 128.1.254.0, subnet mask 255.255.255.0 or CIDR 24
• Each subnet can support 254 hosts• To use an entire subnet for a serial connection would
waste 252 address spaces and we have 20 now – SO…..
Network Subnet ExampleSubnet the Subnet
• Use subnets 128.1.0.0 to 128.1.129.0 for needed subnets with a CIDR of 24
• Subnet subnet 128.1.130.0 using CIDR 30– 128.1.130.0/30– 128.1.130.4/30– 128.1.130.8/30– ………………..– 128.1.130.252/30
Network 2 Subnet Example
• A Network address of 200.10.20.0 is assigned– Subnet with a CIDR of 26
• 200.10.20.0, 200.10.20.64 (62 hosts)
– Subnet subnet 128 with a CIDR of 28• 200.10.20.128, 200.10.20.144, 200.10.20.160 (14 hosts)
– Subnet subnet 200.10.20.176 with a CIDR of 30• 200.10.20.176, 200.10.20.180, 200.10.20.184 (2 hosts)
• Can summarize (aggregate) on– 200.10.20.0/26
Using VLSM
• Variable Length Subnet Masking – allows division of address space based on the size of networks – Start with network requiring the most addresses– Create a subnet mask (use CIDR – Classless
InterDomain Routing – number)– Subnet the subnet as needed to provide address
space required for other subnets• Be logical – start at beginning or end or address space• Addresses must be contiguous to enable route
summarization
Teaching Tips 1
• Make certain students understand subnetting– Provide students with a mix of subnetting problems
using Class A, B, and C addresses and different numbers of bits borrowed to ensure they do understand
• Show relationship of CIDR number of subnet mask
Teaching Tips 2
• Explain reasons for using VLSM
• Explain route aggregation (summarization)
• Explain supernetting
• Show how to summarize using common bits
• Show how to supernet using common bits
Teaching Tips 3
• Show a simple VLSM example using the third octet– First subnet for 255 subnets with 254 hosts;
CIDR = 24– Then subnet one of the subnets for subnets
with CIDR of 28• Subnet 200.16, 200.32, 200.48, etc.
– Then subnet one of the subnets for subnets to use for serial lines and a CIDR of 30
• Subnet 201.4, 201.8, 201.12, 201.16, etc.
Teaching Tips 4
• Show a second example using the fourth octet– Subnet for 8 subnets with a CIDR of 27
• Subnets 0, 32, 64, 96, 128, 160, 192, 224
– Subnet subnet 96, 128, and 160 with a CIDR of 28
• Subnets 96, 112, 128, 144, 160, 176
– Subnet subnets 192 and 224 with a CIDR of 30• Subnets 192, 196, 200, 204, 208, 212, 216, 220, 224,
228, 232, 236, 240, 244, 248, 252
Teaching Tips 5
• Show examples of divided address spaces– Do not use slides – use hard copy and give
students a copy
• Give several problems moving from a very simple problem to a very complex problem– Provide answers for each problem for students to
check as problem is completed