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Subnetting and Variable Length Subnet Masks

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Title: Subnetting and Variable Length Subnet Masks


1
  • Subnetting and Variable Length Subnet Masks

2
Topics
  • Planning Designing
  • Design a simple LAN
  • Design an IP addressing scheme to meet design
    requirements
  • Design a simple internetwork
  • Technology
  • Evaluate TCP/IP communication process and its
    associated protocols
  • Troubleshooting
  • Perform LAN and VLAN troubleshooting
  • Troubleshoot routing protocols
  • Troubleshoot IP addressing and host configuration
  • Troubleshoot a device as part of a working network

3
Benefits of Subnetting
  • Reduced network traffic
  • as a result of segmenting the network
  • reducing broadcast traffic
  • Optimized network performance
  • Simplified management
  • Facilitated spanning of large geographical
    distances
  • by connecting smaller networks together

4
Subnet Masks
  • A 32-bit value that separates the network ID of
    the IP address from the host ID
  • Identifies that part of the host address used for
    the subnet

5
Subnetting Basics
  • An IP address can be separated into network and
    host portions by its class or subnet mask
  • Bits are borrowed from the host portion of the
    IP address to create the subnetworks

6
Creating Subnets
  • Determine the number of network IDs needed,
    typically
  • 1 ID per subnet
  • 1 ID per WAN connection
  • Determine how many hosts IDs per subnet are
    needed, typically
  • 1 ID per host
  • 1 ID per router
  • Create
  • a subnet mask for the entire network
  • a subnet ID for each physical subnet
  • A range of host IDs for each subnet

7
Class Subnet Masks
8
Classless Inter-Domain Routing (CIDR)
  • Does not use class boundaries for distinguishing
    network and hosts portions of IP address
  • Allocates an amount of IP address space to a
    company, home, or customer, based upon customer
    requirements
  • Uses the slash notation (/) to indicate how many
    bits are used for the network portion of the IP
    address also indicates subnet mask value

9
CIDR Values
10
Subnetting Class C Addresses
  • Class C addresses use 24 bits for identifying
    network IDs
  • Bits are borrowed from the host portion to
    subnet, yielding the following subnet masks
    Binary Decimal CIDR 10000000 128
    /25
  • 11000000 192 /26
  • 11100000 224 /27
  • 11110000 240 /28
  • 11111000 248 /29
  • 11111100 252 /30
  • /31 and /32 provide fewer than 2 host IDs

11
Binary Subnetting
  • Determine number of bits used for subnet
  • Example 1000 0000 (1 bit used for subnet)
  • Example 1100 0000 (2 bits for subnet)
  • Determine all possible subnet values for that
    number of bits
  • 00, 01, 10, and 11 are subnets for1100 0000
  • Determine number of hosts per subnet
  • for 1100 0000, six bits define 26 2
    hosts/subnet
  • Determine broadcast address and host ID range
  • Broadcast address has all 1s in host portion

12
Subnets 00 and 01
13
Subnets 10, 11
14
Subnetting Class C Addresses Fast Method
Answer Five Simple Questions
15
How Many Subnets?
  • 2x number of subnets
  • x is the number of masked bits (1s)
  • For example, if 2 bits are used to identify the
    subnet (1100 00002 19210), there are 22 or 4
    subnets

16
How Many Hosts Per Subnet?
  • 2y - 2 number of hosts per subnet
  • y is the number of unmasked bits (0s)
  • For example, if 2 bits are used for the network
    ID, then 6 bits are left for host IDs
  • Thus the number of hosts per subnet is 26-2 or 62.

17
What Are The Valid Subnets?
  • 256 - subnet mask block size and subnet base
    value (Critical values know these and everything
    else is simple)
  • For example 256 - 19264. 64 is the first
    subnet.
  • The next subnet would be the base value plus the
    block size or 64 64 128, the second subnet

18
Subnet Broadcast Addresses
  • The broadcast address is determined by setting
    all host bits to 1
  • This is the number immediately preceding the next
    subnet

19
Valid Host IDs
  • Valid hosts are the values between the subnets,
    omitting all 0s and all 1s IDs

20
Variable Length Subnet Masks (VLSM)
  • Some segments do not need as many IDs as do other
    segments
  • VLSM conserves address space by allotting the
    minimum number of IDs to each segment
  • Like the name implies this is accomplished by
    using masks of different lengths

21
Routing Protocols that Support VLSM
  • RIPv2, EIGRP, and OSPF accommodate VLSM by
    including a field for subnet information
  • RIPv1 and IGRP do not
  • Protocols that support VLSM are said to be
    classless
  • Protocols that do not support VLSM are called
    classful

22
Same Mask Subnetting
23
Applying VLSM
24
Implementing VLSM
  • Relate number of hosts needed to block sizes
  • Create VLSM table using VLSM Worksheet to avoid
    overlap

25
VLSM Worksheet
26
VSLM, Example 1
27
VLSM Worksheet for Example 1
28
VLSM, Example 2
29
VLSM Worksheet for Example 2
30
What are the IP addresses for the Fast Ethernet
and Serial Interfaces?
31
Answer
32
Summarization or Route Aggregation
  • Advertises many networks as one
  • Reduces size of routing tables
  • Determine block size and summary mask

33
Troubleshooting
  • The following examples provide practice at
    troubleshooting subnet addressing problems
  • General trouble shooting steps
  • ping localhost
  • ping local IP address
  • ping default gateway
  • ping remote server
  • Consider DNS (ping host names and IP addresses)

34
Why Cant the Sales LAN Contact Server A?
35
Why Cant Hosts in the Sales LAN Contact Server B?
36
What is the Host Address?
37
Which Addresses Can Be Assigned to the Hosts?
38
Which IP Addresses Can Be Assigned?
39
Which Class C Mask Should Be Used?
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