Transcript Slide 1

The network mask is represented as a 32 bit string. The network portion of
the IP address is represented as a string of ones starting at the highest order
bit position. The host portion of the mask is represented by a string of zeros
starting at the lowest order bit.
Ex:
255:255:255:0 Binary 11111111 11111111 11111111 00000000
Since there are 24 1’s in this mask this is known as a 24 bit mask.
An IP subnet is specified by the address of the network portion of the subnet
and the host portion as zeros. The length of the mask is appended, after a ‘/’
character, to the subnet address.
Ex: 144.167.26.0/24
Binary: 10010000 10100111 00011010 00000000
Mask:
11111111 11111111 11111111 00000000
---------Network -------------------- ^---Host---
Example addresses:
Subnet: 144.167.26.0/24
IP:
144.167.26.26
Binary
10010000 10100111 00011010 00011010
Mask:
11111111 11111111 11111111 00000000
IP&MASK
IP:
144.167.26.55
IP&MASK
10010000 10100111 00011010 00000000
Binary
1001000 10100111 00011010 00110111
1001000 10100111 00011010 00000000
Since the IP&MASK calculations are identical, these two addresses are on the
same subnet (local), If the target IP was not in the host’s ARP table the host
would generate an ARP request for the target IP and then send an ethernet packet
directly to the destination.
Example addresses:
Subnet: 144.167.26.0/24
IP:
144.167.26.26
IP&MASK
IP:
144.167.27.26
IP&MASK
Binary
Mask:
10010000 10100111 00011010 00011010
11111111 11111111 11111111 00000000
10010000 10100111 00011010 00000000
Binary: 10010000 10100111 00011011 00011010
10010000 10100111 00011011 00000000
Since the IP&MASK outcomes for these two addresses are not equal, they are
not on the same subnet. (remote)
In this case the IP configuration would be examined for the address of the default
gateway. The packet would be sent to the default gateway for routing. If the
default gateway’s IP address was not in the ARP table of the host, and ARP
would be issued first then the MAC address of the default gateway would be
used to forward the packet to the router.
Addresses not on byte boundries may be more difficult to determine by
examination, but the technique is the same:
Subnet: 144.167.16.0/23 Binary: 10010000 10100111 00010000 00000000
Mask:
11111111 11111111 11111110 00000000
-----Network------------------------^----Host----
Ex:IP:
144.167.17.16
Binary
10010000 10100111 00010001 00010000
Notice the the 9th bit, which has changed between the subnet address and the
Target IP, is in the host portion of the address and is thus local to the subnet.
ExIP:
144.167.18.16
Binary
10010000 10100111 00010010 00010000
This time the 10th bit has changed and this is in the network portion of the
address so this target address is not local to the subnet.