Mahmmoud Mahdi
Allows you to take one larger network and break it into a bunch of smaller networks. benefits:
Reduced network traffic
Optimized network performance Simplified management
Facilitated spanning of large geographical
distances
192.168.2.1
192.168.3.1
192.168.3.2
192.168.2.2
192.168.3.3
A 32-bit value that allows the recipient of IP packets to distinguish between: The 1s in the subnet mask represent: Not all networks need subnets:
meaning they use the default subnet mask. These default masks cannot change.
Class A network, you cant change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Class B: start with 255.255.0.0 Class C: start with 255.255.255.0.
The network ID portion of the IP address. The host ID portion of the IP address. the positions that refer to the network or subnet
addresses.
Class Format
A B C Network. Node. Node. Node Network. Network. Node. Node Network. Network. Network. Node
Default Subnet Mask
255.0.0.0 255.255.0.0 255.255.255.0
Class B address Network address 130.57. Host address 1.56
1000 0010. 0011 1001.
0000 0001. 0011 1000
Class B address with a subnet address Network address 1000 0010. 0011 1001. Subnet Host address
0000 0001. 0011 1000
To create subnet:
Take bits from the host portion of the IP address
and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits available for defining hosts.
Determine the number of required network IDs:
One for each subnet One for each wide area network connection
Determine the number of required host IDs per subnet:
One for each TCP/IP host One for each router interface
Based on the above requirements, create the following:
One subnet mask for your entire network A unique subnet ID for each physical segment A range of host IDs for each subnet
21 = 2 22 = 4
28 = 256 29 = 512
23 = 8
24 = 16 25 = 32 26 = 64 27 = 128
210 = 1,024
211 = 2,048 212 = 4,096 213 = 8,192 214 = 16,384
192.168.2.1 255.255.255.0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
192.168.2.1/24 (CIDR notation)
1 network 28-2=254 addresses
192.168.2.1 255.255.255.224
192.168.2.32 192.168.2.64 192.168.2.96 192.168.2.128
192.168.2.128 192.168.2.160 192.168.2.192 192.168.2.234
23=8subnets
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 192.168.2.1/27 (CIDR notation)
network
25-2=30 addresses
CIDR creates a hierarchical addressing structure by breaking the network address space into CIDR blocks CIDR notation:
describes a network by specifying the base
address and the number of bits used for the network portion i.e., 10.0.0.0/8
a network using 24 bits for host numbering a maximum of 224 -2 possible host addresses.
Method that ISPs use to allocate a number of addresses to a company, a homea customer. 192.168.10.32/28:
Telling you what your subnet mask is.
The slash notation (/)
how many bits are turned on (1s).
Keep in Mind:
the largest subnet mask available (regardless of the
class of address) can only be a /30.
youve got to keep at least 2 bits for host bits.
Example:
A Class A default subnet mask:
which is 255.0.0.0
This means that the first byte of the subnet mask is all ones (1s), or 11111111.
A slash notation:
you need to count all the 1s bits to figure out your mask.
The 255.0.0.0 is considered a /8
A Class B default mask:
255.255.0.0
which is a /16 because 16 bits are ones (1s): 11111111.11111111.00000000.00000000.
Network Class Class A Class B
Starting Address 0.0.0.0 128.0.0.0
Ending Address 127.255.255.255 191.255.255.255
CIDR Block /8 /16
Bitmask 255.0.0.0 255.255.0.0
Class C
Class D (multicast) Class E (reserved)
192.0.0.0
224.0.0.0 240.0.0.0
223.255.255.255
239.255.255.251 255.255.255.255
/24
/4 /4
255.255.255.0
Subnet Mask 255.0.0.0 255.128.0.0 255.192.0.0 255.224.0.0 255.240.0.0 255.248.0.0 255.252.0.0
CIDR Value /8 /9 /10 /11 /12 /13 /14
Subnet Mask 255.255.240.0 255.255.248.0 255.255.252.0 255.255.254.0 255.255.255.0 255.255.255.128 255.255.255.192
CIDR Value /20 /21 /22 /23 /24 /25 /26
255.254.0.0 255.255.0.0
255.255.128.0 255.255.192.0 255.255.224.0
/15 /16
/17 /18 /19
255.255.255.224 255.255.255.240
255.255.255.248 255.255.255.252
/27 /28
/29 /30
The /8 through /15
The /16 through /23 The /24 through /30
can only be used with Class A network addresses. can be used by Class A and B network addresses. can be used by Class A, B, and C network addresses.
This is a big reason why most companies use Class A network addresses.
Since they can use all subnet masks, they get the
maximum flexibility in network design.
Class C address, only 8 bits are available for defining
the hosts. Class C subnet masks can be the following:
Binary 00000000 10000000 11000000 11100000 11110000 11111000 11111100 Decimal =0 = 128 = 192 = 224 = 240 = 248 = 252 CIDR /24 /25 /26 /27 /28 /29 /30
We cant use a /31 or /32 because we have to have at least 2 host bits for assigning IP addresses to hosts.
Answer five simple questions:
How many subnets does the chosen subnet mask produce? How many valid hosts per subnet are available? What are the valid subnets? Whats the broadcast address of each subnet? What are the valid hosts in each subnet?
2x= number of subnets. x is the number of masked bits, or the 1s.
For example: in 11000000, the number of 1s gives us 22 subnets. In this example, there are 4 subnets.
2y 2 = number of hosts per subnet. y is the number of unmasked bits, or the 0s.
Example: in 11000000, the number of 0s gives us 26 2 hosts. In this example, there are 62 hosts per subnet. You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts.
256 subnet mask = block size, or increment
number. Ex: 256 192 = 64:
The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reach the subnet mask value.
0, 64, 128, 192.
The broadcast address is always the number
right before the next subnet. Example:
The 0 subnet has a broadcast address of 63
Because: the next subnet is 64.
The 64 subnet has a broadcast address of 127
because the next subnet is 128. And so on.
Remember: the broadcast address of the last
subnet is always 255.
The numbers between the subnets, omitting the
all 0s and all 1s.
its always the numbers between the subnet address and the broadcast address.
Example: if 64 is the subnet number and 127 is the broadcast address, then 65126 is the valid host range.
Subnet the Class C network address 255.255.255.128 (/25)
192.168.10.0 = Network address
255.255.255.128 = Subnet mask
How many subnets?
Since 128 is 1 bit on (10000000), the answer would be 21 = 2.
How many hosts per subnet?
We have 7 host bits off (10000000), so the equation would be 27 2 = 126 hosts.
What are the valid subnets?
256 128 = 128. start at zero and count block size, so subnets are 0, 128.
Whats the broadcast address for each subnet?
The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the zero subnet, the next subnet is 128, so the broadcast of the 0 subnet is 127.
What are the valid hosts?
These are the numbers between the subnet and broadcast address
Subnet First host Last host Broadcast
192.168.10.0 192.168.10.1
192.168.10.128 192.168.10.129
192.168.10.126 192.168.10.254 192.168.10.127 192.168.10.255
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