Pv4 supports three different types of addressing modes

Class a subnets in class a only the first octet is

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Class A Subnets
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In Class A, only the first octet is used as Network identifier and rest of three octets are used to be assigned to Hosts (i.e. 16777214 Hosts per Network). To make more subnet in Class A, bits from Host part are borrowed and the subnet mask is changed accordingly. For example, if one MSB (Most Significant Bit) is borrowed from host bits of second octet and added to Network address, it creates two Subnets (2 1 =2) with (2 23 -2) 8388606 Hosts per Subnet. The Subnet mask is changed accordingly to reflect subnetting. Given below is a list of all possible combination of Class A subnets:
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In case of subnetting too, the very first and last IP address of every subnet is used for Subnet Number and Subnet Broadcast IP address respectively. Because these two IP addresses cannot be assigned to hosts, sub-netting cannot be implemented by using more than 30 bits as Network Bits, which provides less than two hosts per subnet. Class B Subnets By default, using Classful Networking, 14 bits are used as Network bits providing (2 14 ) 16384 Networks and (2 16 -2) 65534 Hosts. Class B IP Addresses can be subnetted the same way as Class A addresses, by
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borrowing bits from Host bits. Below is given all possible combination of Class B subnetting: Class C Subnets Class C IP addresses are normally assigned to a very small size network because it can only have 254 hosts in a network. Given below is a list of all possible combination of subnetted Class B IP address:
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IPv4 - VLSM Internet Service Providers may face a situation where they need to allocate IP subnets of different sizes as per the requirement of customer. One customer may ask Class C subnet of 3 IP addresses and another may ask for 10 IPs. For an ISP, it is not feasible to divide the IP addresses into fixed size subnets, rather he may want to subnet the subnets in such a way which results in minimum wastage of IP addresses. For example, an administrator have 192.168.1.0/24 network. The suffix /24 (pronounced as "slash 24") tells the number of bits used for network address. In this example, the administrator has three different departments with different number of hosts. Sales department has 100 computers, Purchase department has 50 computers, Accounts has 25 computers and Management has 5 computers. In CIDR, the subnets are of fixed size. Using the same methodology the administrator cannot fulfill all the requirements of the network. The following procedure shows how VLSM can be used in order to allocate department-wise IP addresses as mentioned in the example. Step - 1 Make a list of Subnets possible.
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Step - 2 Sort the requirements of IPs in descending order (Highest to Lowest). Sales 100 Purchase 50 Accounts 25 Management 5 Step - 3 Allocate the highest range of IPs to the highest requirement, so let's assign 192.168.1.0 /25 (255.255.255.128) to the Sales department. This IP subnet with Network number 192.168.1.0 has 126 valid Host IP addresses which satisfy the requirement of the Sales department. The subnet mask used for this subnet has 10000000 as the last octet.
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  • Winter '17
  • Prof. David
  • IP address, IP addresses

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