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21-IP addressing
Dr. John P. Abraham
Professor
UTPA
Chapters 19, 20
• I have already covered these concepts elsewhere.
Addressing
• Physical
• Logical
IP address
• 32 bits– Hierarchy – prefix and suffix– Prefix identifies the network – given by IANA
In classful addressing, the network address (the first address in the block) is the one that is assigned to the organization. The range of
addresses can automatically be inferred from the network address.
– Suffix identifies the computer –given locally– No two computers can have the same public IP
Octets
• IP address is written in 4 octets with separating dots.
• 10000000.00001011.00000011.00011111
• 128.11.3.31
• One of the UTPA CS address is:
• 10000001.01110001.10000010.01101110
• What is this address in decimal notation?
Classful IP addressing
• Divided IP address space into three primary classes A, B, C and also there exist class D (multicasting) and E.
• First four (MSB) bits will determine its class
ClassLeadingBits
Size of NetworkNumber Bit field
Size of RestBit
field
Numberof
Networks
Addresses
per Network
Start addres
s
End address
Class A 0 8 24 128 (27) 16,777,216 (224)
0.0.0.0 127.255.255.255
Class B 10 16 16 16,384 (214)
65,536 (216)
128.0.0.0
191.255.255.255
Class C 110 24 8 2,097,152 (221)
256 (28) 192.0.0.0
223.255.255.255
Class D (multicast)
1110
not defined
not defined
not defined
not defined
224.0.0.0
239.255.255.255
Class E (reserved)
1111
not defined
not defined
not defined
not defined
240.0.0.0
255.255.255.255
Finding the address class
Figure 4.5 Finding the class in decimal notation
Find the class of each address:
a. 227.12.14.87 b.193.14.56.22 c.14.23.120.8d. 252.5.15.111 e.134.11.78.56
Solutiona. The first byte is 227 (between 224 and 239); the class is D.b. The first byte is 193 (between 192 and 223); the class is C.c. The first byte is 14 (between 0 and 127); the class is A.d. The first byte is 252 (between 240 and 255); the class is E.e. The first byte is 134 (between 128 and 191); the class is B.
Subnet Addressing
• The network address is the beginning address of each block. It can be found by applying the default mask to any of the addresses in the block (including itself). It retains the netid of the block and sets the hostid to zero.
Range of Addresses
• We often need to find the range of address given to an organization.– Find the number of addresses if first and last
are given: 146.102.29.0 and 146.102.32.255– There are 256 addresses in the last octet. For
the third octet 32. There are 256 addresses in 29, 30, 31 (and 32) for a total of 256*4 = 1024 addresses.
Finding the first and last address given one IP address and the mask
• To find the first IP address– Keep the net portion the same, and turn all
host portions to 0.
• To find the last address: – Keep the net portion the same, and turn all
host portion to 1s.
Find the first and last IP
• Given 73.22.17.25 and mask of 255.0.0.0. find the first and last addresses.– 73.0.0.0 and 73.255.255.255– How many addresses are in the block?
16,777,216
Finding the network portion
• Given the IP address and mask, AND
• Default masks are: – For class A 255.0.0.0– For class B 255.255.0.0– For class C 255.255.255.0
• However you can create any mask you want based on some rules.
Finding first and last addressrevisited
• Find the First address in the block– IP address AND network mask
• Find the last address in the block– IP address OR complement of network mask
Given the address 23.56.7.91, find the beginning address (network address).
Example 12
SolutionThe default mask is 255.0.0.0, which means that only the first byte is preserved and the other 3 bytes are set to 0s. The network address is 23.0.0.0.
What is the subnetwork address if the destination address is 200.45.34.56 and the subnet mask is 255.255.240.0?
Example 15
SolutionWe apply the AND operation on the address and the subnet mask.
Address ➡ 11001000 00101101 00100010 00111000
Subnet Mask ➡ 11111111 11111111 11110000 00000000
Subnetwork Address ➡ 11001000 00101101 00100000 00000000.
Figure 4.25 Comparison of a default mask and a subnet mask
Table 4.3 Special addressesTable 4.3 Special addresses
Private IPs
CIDR notation
• Classless Inter-Domain routing (CIDR)
CIDR
• CIDR is an alternative to traditional IP subnetting that organizes IP addresses into subnetworks independent of the value of the addresses themselves. CIDR is also known as supernetting as it effectively allows multiple subnets to be grouped together for network routing.
• Ddd.ddd.ddd.ddd/m 192.5.48.69/26
CIDR addressing
• In the address 167.199.170.82/27 the network mask is 255.255.255.224 (27 ones and five zeros).
• The suffix length of 5, meaning there can be 32 host addresses.
• We can find the first address by ANDING the IP with mask. We can find the last address by ORing the address with the complement of the mask.
CIDR
• Prefix length is given after the slash• 230.8.24.56/16 gives block of 230.8.0.0 to 230.8.255.255• To find the first address AND the mask.• To find the last address, find the complement of the mask, then OR.
CIDR example
• One of the address in a block is 17.63.110.114/24. Find the number of address, the first address, and the last address of the block.
17 63 110 114
255 255 255 0 AND
17 63 110 0 IS THE FIRST ADDRESS
---
17 63 110 114
0 0 0 255 OR
17 63 110 255 IS THE LAST ADDRESS
CIDR example• 110.23.120.14/20. Find the first and last
address
110 23 120 14
255 255 240 0 AND
256 23 112 0
110 23 120 14
0 0 15 255 OR
110 23 127 255 LAST ADDRESS
Supernetting
• Class C network addresses are still available, but one may not be enough for an organization.
• In supernetting we can combine several class C blocks to create a larger range of addresses.
• If an organization needs 1000 addresses, we can join 4 class C blocks.
Supernet masks
• For Supernetting we go backwards with the subnet mask. In this case we have 2 less 1s than the default subnet mask.
• The default subnet mask for class C is 255.255.255.0. In this case we go 2 bits to the left, or 255.255.252.0.
• In superneting, the number of blocks to combine needs to be a power of 2.
