vlsm variable length subnet masks
DESCRIPTION
VLSM Variable Length subnet masks. Subnetting the subnet. IP Address Design Example. Company with 200 hosts Assigned class C network 195.10.1.0/24 Want 6 different LANs Subnet class C network using mask 255.255.255.224 (/27) - PowerPoint PPT PresentationTRANSCRIPT
Subnetting the subnet
RD-CSY2001-2008/09
Company with 200 hosts Assigned class C network 195.10.1.0/24 Want 6 different LANs Subnet class C network using mask 255.255.255.224
(/27) Whole network has the same subnet mask or supports equal
number of hosts on all the sub-networks◦ Is this a problem? ◦ Why?◦ What should be done to solve it ?
RD-CSY2001-2008/09
VLSM Subnets a subnet◦VLSM divides a network into subnets of various sizes to
prevent wasting IP addresses Uses up wasted addresses Conserves address space on WAN links
Point-to-point link only needs 2 addresses With VLSM point-to-point links can use a /30 mask
RD-CSY2001-2008/09
Variable Length Subnet Masking allows division of address space based on the size of networks ◦Start with network requiring the most addresses◦Create a subnet mask
(use CIDR – Classless Inter Domain Routing)◦Subnet the subnet as needed to provide address space
required for other subnets Be logical – start at beginning or end or address space Addresses must be contiguous to enable route
summarization
RD-CSY2001-2008/09
Question If an ISP has a 200.1.1.0 Network, he wants to give
assign to 5 customers who need 20, 12, 6, 25, 18 hosts.◦ What should the subnet mask be?◦ What is the address range in each subnet?◦ What should the routing table entries be?◦ Can it be done efficiently to conserve IP addresses?
RD-CSY2001-2008/09
RD-CSY2001-2008/09
RD-CSY2001-2008/09
Address format <IP address/prefix P>. The prefix denotes the upper P bits of the IP address. Can be used to specify arbitrary blocks of addresses◦ Say an ISP has 192.5.48.0, 192.5.49.0,192.5.50.0,192.5.51.0
then◦ the IP address advertised will be 192.5.48.0/22
An ISP can obtain a block of addresses and partition this further to its customers
Say an ISP has 200.8.4/24 address (256 addresses)◦He has another customer who needs only 4 addresses from
200.8.4.24 then that block can be specified as 200.8.4.24/30
RD-CSY2001-2008/09
RD-CSY2001-2008/09
Net A: 14 HOSTS
Net B: 28 HOSTS
Net D: 7HOSTS
Net E: 28 HOSTS
Net C: 2 HOSTS
Router A Router B
Given IP address 204.15.5.0/24, use VLSM to create a subnetting scheme
Please try other questions on VLSM, available on module web page
RD-CSY2001-2008/09