Transcript From Subnetting to VLSM - YSU Computer Science

From Subnetting
to 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
• Class B
• Class C
0-126
128-191
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 Routing
Address 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 Addressing
RFC 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
VLSM
Variable 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
– 172.17.0.0
– 172.18.0.0
10101100.000100 00.0.0
10101100.000100 01.0.0
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
– 10.21.0.0
– 10.22.0.0
00001010.000101 00.0.0
00001010.000101 01.0.0
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
200.10.11.0
200.10.12.0
200.10.13.0
11001000.00001010.00001010.0
11001000.00001010.00001011.0
11001000.00001010.00001100.0
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
200.10.102.0
200.10.103.0
200.10.104.0
11001000.00001010.11001001.0
11001000.00001010.11001010.0
11001000.00001010.11001011.0
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 Example
Subnet 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