modul_04_subneting ipv4
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Layer 3 Devices
Routers
Interconnect network segments or entirenetworks.
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.
Determines best path for data on an internetwork.
A.K.A Layer 3 Switching
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Path Determination
The process the router uses to choose the next hop
in the path the packet travels.
The router uses the network address to identify thedestination network of a packet within an
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internetwork. IP addresses can be assigned by a network
administrator or automatically (dynamically.)
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Layer 3 Packet/Datagram
VERS HLEN Service Type Total Length
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Time to Live Protocol Header Checksum
Source IP Address
Destination IP AddressIP Options (if any) Padding
Data
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Network Layer Addresses
Network layer addresses are 32 bits long.
They are represented as four octets in dotted
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233.14.17.0ec ma ormat.
The IP address has two components:
The Network ID
The Host Id
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Layer 3 Addresses
Network ID
Assigned by ARIN(www.arin.net)
Identifies the network to
Host ID
Assigned by a networkadministrator.
Identifies the specific
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w c a ev ce sattached.
May be identified by one,two, or three of the first
three octets.
ev ce on a ne wor .
May be identified by one,two, or three of the lastthree octets.
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IP Addresses
32 bit address represented as 8 bit dotted decimals.
Different class addresses reserve different amounts of
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.
Class A N H H H
Class B N N H H
Class C N N N H
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Number of Hosts
Maximum number of hosts vary for each class.
Class A has 16,777,214 available hosts (224 –2)
Class B has 65,534 available hosts (216 –2)
Class C has 254 available hosts (28 –2)
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The first address in each network is reserved for thenetwork address and the last address is reserved for
the broadcast address.
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Classes
How do you know what class an IP address
is in?
If the first octet is between:
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– s a c ass a ress 128-191 it is a class B address
192 – 223 it is a class C address
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Recognizing Class in Binary
Format
128s place 64s place 32s place
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Class B 1 0
Class C 1 1 0
Initial bitpattern infirst octet
of IPaddress.
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Reserved Addresses
Broadcast Address – is used to send data to all of
the devices on a network. Broadcast IP addresses end with binary 1s in the
host part of the address.
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Class B Broadcast Address example: 176.10.255.255
(Remember decimal 255 = binary 11111111)
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Basics of Subnetting
Subnetworks are smaller divisions of
networks.
They provide addressing flexibility.
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. . . su nets Subnet addresses are assigned locally,
usually by a network administrator.
Subnets reduce a broadcast domain.
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Subnet Addresses
Include Class A, B, or C network portion plus
a subnet field and a host field. Bits are borrowed from the host field and are
desi nated as the subnet field.
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Network Subnet Host
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How many bits can I borrow?
Size of Host Field Maximum # of borrowed bits
The minimum number of bits you can borrow is two.
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Class A24 22
Class B 16 14
Class C 8 6
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Default Subnet Masks
Class A 255.0.0.0
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Class B 255.255.0.0
Class C 255.255.255.0
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Calculating a Subnet
We will subnet the IP address:
223.14.17.0
What class IP address is this?
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ass
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Step #1
Determine the default subnet mask
Class C default subnet mask:
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255.255.255.0
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Step #2
Determine the number of subnets needed
and hosts on each to determine how manybits to borrow from the host ID.
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13 subnets
10 hosts on each subnet
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Step #3
Figure the actual number of subnets and
hosts by borrowing bits from host ID.
Let’s see how many subnets and hosts we
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.
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Step #3 continued…
223.14.17.0
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X X X X H H H H
16 possiblesubnets
16 possiblehosts for each
subnet
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Step #3 continued…
We get 16 possible subnets and 16 possible hosts for each subnet because:
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leaving you with 24 or 16 possible combinations.
The same goes for the 4 leftover host bits.
Important: There are only 14 available subnets and hosts on each subnet. Why?
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Step #3 continued…
Because you cannot use the first and last
subnet.
Because you cannot use the first and last
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.
For each, one is the broadcast address andone is the network address.
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Step #4
Determine the subnet mask.
223.14.17.0
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X X X X H H H H
Where X represents the borrowed bits forsubnetting.
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Step #4 continued…
Add the place values of X together to get the
last octet decimal value of the subnet mask.
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128 + 64 + 32 + 16 = 240
The subnet mask is: 255.255.255.240
The subnet mask is used to reveal thesubnet and host address fields in IP
addresses.
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Step 5
Determine the ranges of host addresses for
each subnet.
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Step 5 continued…
Subnet # Subnet Bits Host Bits In Decimal1 0000 0000-1111 .0 -.15
2 0001 0000-1111 .16 - .31
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- . - .
4 0011 0000-1111 .48 - .63
5 0100 0000-1111 .64 - .79
6 0101 0000-1111 .80 - .95
7 0110 0000-1111 .96 - .111
8 0111 0000-1111 .112 - .127
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Step 5 continued…
Subnet # Subnet Bits Host Bits In Decimal9 1000 0000-1111 .128 -.143
10 1001 0000-1111 .144 - .159
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- . - .
12 1011 0000-1111 .176 - .191
13 1100 0000-1111 .192 - .207
14 1101 0000-1111 .208 - .223
15 1110 0000-1111 .224 - .239
16 1111 0000-1111 .240 - .255
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Step 5 continued…
There are 16 possible subnets.
There are 16 possible hosts on each subnet.
That equals 256 possible hosts.
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What are our available subnets? What are our available hosts on each
subnet? Why?????
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Figuring Subnet
Network Addresses
Step #1: Change the IP host address to
binary. Step #2: Change the subnet mask to binary.
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combine the two. Step #4:Convert the network binary address
to dotted decimal.
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Figuring Subnet
Network Addresses
IP Host 172.16.2.120Subnet Mask 255.255.255.0
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. . .
11111111.11111111.11111111.00000000
10101100.00010000.00000010.00000000
172.16.2.0
This is the subnet network address. It is the lowestnumbered address on the subnet network. It can help
determine path.
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