VLSM: Variable-Length Subnet Masking for IP Addressing, Study notes of Architecture

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Networking Architecture

Lecture4: VLSM

Lecture: Dr.Suad Abdulelah Alasadi

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.

• 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.

Networks Address Allocating Methods

VLSM/Classless addressing

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

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 2^5 =32–2. Only 30 usable host addresses are available

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/.

4. Find the host range, and broadcast address

for each subnet

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.

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.

Methods of Assigning IP Addresses

Thank You