Addressing Activities
ƒ 6.5.4
Calculating network address from host IP and
subnet mask.
ƒ 6.5.5
Calculating the number of Hosts (note you only
need to know the mask).
Addressing the Network
– Examples
ƒ 6.5.6
Calculating host IP range, broadcast address
from network address and mask.
Network Fundamentals – Chapter 6
ƒ 6.5.7, 6.5.8
Packet Tracer
ƒ 6.5.3.2 to 6.5.3.5 VLSM example
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Network Classes
Class First octet
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6.5.3.1 Classful Subnetting
Network and Default subnet
Host bits
mask
Hosts per
network
N.H.H.H /8
16,777,214
A
1 to 126
255.0.0.0
B
128 to 191 N.N.H.H /16 255.255.0.0
C
192 to 223 N.N.N.H /24 255.255.255.0
65,534
ƒ Classful network address: 192.168.20.0 /24 (Class C addr)
ƒ Require 7 subnets, i.e. borrow 3 bits,
ƒ Subnetmask: 24 + 3 bits = /27
2^3 = 8 subnets
which is 255.255.255.224
254
ƒ This leaves 30 – 27 = 5 bits for host bits, 2^5 – 2 = 30 hosts
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Classful Subnetting
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Classful Subnetting
ƒ A class A, B or C network is used with a custom
mask
ƒ The same mask is used on all interfaces making
the subnets all the same size
ƒ What subnet mask should be
used?
ƒ How many hosts can each
subnet connect?
ƒ This is inefficient use of addresses
ƒ The WAN links only require 2 addresses, so 28 are
6
not used, limiting future growth
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ƒ The number of bits borrowed is determined by
1. Number of subnets required
2. Max number of hosts on a subnet
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Variable Length Subnet Masking (VLSM)
Subnetting a Subnet
ƒ Using more than one subnet mask on a
network is called Variable Length Subnet
Masking
ƒ Use one of the subnets for the WAN links
say 192.168.20.192 /27
ƒ Require 2 host IPs per subnet
ƒ VLSM is a more efficient way of assigning
addresses than classful subnetting
2^2 – 2 = 2, so use /30 mask
ƒ With VLSM the subnets do not have to be all
the same size, so fewer addresses are wasted
ƒ Borrow 3 bits gives 2^3 = 8 subnets
ƒ In particular, a WAN link only requires a subnet
with 2 addresses, so use /30 subnets for these
ƒ Subnets 4, 5 and 7 are now available for
growth, plus 5 subnets for additional WAN links
192.168.20.192/30
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Subnetting a Subnet
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Variable Length Subnet Masking (VLSM)
ƒ VLSM is a more efficient way of assigning
addresses than classful subnetting, but it can
only be used with modern routing protocols
ƒ Determine how many addresses required for
each subnet,
number of hosts + router interface
ƒ Work out largest subnet first, WAN links last
ƒ How many bits must be borrowed for the
subnet?
Number of hosts = 2 ^ (no. of bits) - 2
ƒ This determines the size of the subnet
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6.5.3.2 Scenario
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6.5.3.2 Scenario
ƒ This scenario has the following requirements:
AtlantaHQ 58 host addresses
PerthHQ 26 host addresses
SydneyHQ 10 host addresses
CorpusHQ 10 host addresses
WAN links 2 host addresses (each)
ƒ Design an addressing scheme using the
network address 192.168.15.0 /24
ƒ Classful subnetting will not work. Why not?
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ƒ Use VLSM instead.
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6.5.3.2: Given address 192.168.15.0 /24
Num of
Host
Subnet
Broadcast
Prefix
A
IP’s
58
bits
6
address
192.168.15.0
address
192.168.15.63
/26
B
26
5
192.168.15.64
192.168.15.95
/27
C
10
4
192.168.15.96
192.168.15.111
/28
D
10
4
192.168.15.112
192.168.15.127
/28
W1
2
2
192.168.15.128
192.168.15.131
/30
W2
2
2
192.168.15.132
192.168.15.135
/30
W3
2
2
192.168.15.136
192.168.15.139
/30 13
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