Networking Architecture

Lecture4: VLSM

Lecture: Dr.Suad Abdulelah Alasadi

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Networks Address Allocating Methods

• There are two methods available for allocating addresses to an internetwork:

1. use a non-VLSM (FLSM)approach, where all subnets use the same prefix length and the same number of host bits for each subnet.

2. You can use variable-length subnet masking (VLSM), where you assign the prefix and host bits to each network based on the number of hosts in that network.

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Networks Address Allocating Methods

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• using variable-length subnet mask (VLSM), was designed to maximize addressing efficiency.

• VLSM is a practice associated with classless addressing.

• When identifying the total number of hosts using traditional subnetting (FLSM), you allocate the same number of addresses for each subnet, while in VLSM the subnet mask is different and the number of host bit for each subnet is also different.

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Networks Address Allocating Methods

VLSM/Classless addressing

• Given an IP address 192.168.1.0/24, address the internetwork using VLSM

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VLSM/Classless addressing

• The following are the host requirements:

■ Network A: 14 hosts

■ Network B: 28 hosts

■ Network C: 2 hosts

■ Network D: 7 hosts

■ Network E: 28 host

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VLSM/Classless addressing

1. begin the process by subnetting for the largest host requirement first.

In this case, the largest requirements are for network B and network E, each with 28 hosts.

2. Apply the formula usable hosts = 2n–2.

For networks B and E, 5 bits are borrowed from the host portion and the calculation is 25=32–2. Only 30 usable host addresses are available

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VLSM/ Classless addressing

3. Find the subnet mask for each subnet

in network B,E, we need 5 bit for address the host, 3 bit borrowed for address the subnetwork.

The subnet mask will be 192.168.1.0/27 .

4. Find the host range, and broadcast address for each subnet

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VLSM/ Classless addressing • The network address , host range, and subnet mask for each

subnet are :

• Network B will use subnet 0: 192.168.1.0/27, host address range 1 to 30

• Network E will use Subnet1:192.168.1.32/27, host address range 33 to 62

• Network A will use subnet 0: 192.168.1.64/28, host address range 65 to 78

• Network D will use subnet 1: 192.168.1.80/28, host address range 81 to 94

• Network C will use subnet 1: 192.168.1.96/30, host address range 97 to 98.

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Methods of Assigning IP Addresses • Address assignment includes assigning an IP address, a default

gateway, one or more domain name servers that resolve names to IP addresses, time servers, and so forth.

• There are two basic IP address assignment strategies for a device (Static and dynamic )

Static :

IP address is statically assigned to a system.

The network administrator manually configures the following: IP address.

Default gateway

Name servers

• Server , Printer are examples of static assignment of ip address.

• Network devices such as routers and switches typically have static addresses.

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Methods of Assigning IP Addresses

• IP addresses are dynamically assigned to the end systems.

Dynamic Host Configuration Protocol (DHCP) is the newest and provides the most features for assign IP automatically.

DHCP is used to provide dynamic IP address allocation to hosts. DHCP uses a client/server model.

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Steps to allocate IP address using DHCP:

Step 1 The host sends a DHCPDISCOVER broadcast message to locate a DHCP server.

Step 2 A DHCP server offers configuration parameters such as an IP address, a MAC address, a domain name, a default gateway, and a lease for the IP address to the client in a DHCPOFFER unicast message.

Step 3 The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message.

Step 4 The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.

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Methods of Assigning IP Addresses

Methods of Assigning IP Addresses

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Thank You

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