Hey guys, I'm studying for the 70-291 and I'm having a hard time understanding VLSM's. I think I have it now, but I just need someone to confirm my understanding. Below is an example Class B default network that I broke up using my understanding of VLSM. If someone could confirm if it is correct, and if not, point out my mistakes and help to explain where I went wrong, I would greatly appreciate it. Thanks! Default Network 150.1.0.0/16 - original network 150.1.0.0/24 (255.255.255.0) makes 256 subnets with 256 hosts per subnet 150.1.1.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) 150.1.2.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) 150.1.3.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) 150.1.4.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) ..... ..... ..... 150.1.56.0/30 (255.255.255.252) - 64 subnets with 4 addresses per subnet (2 assignable, 1 net id, 1 broadcast) 150.1.56.4/30 150.1.56.8/30 150.1.56.12/30 150.1.56.16/30 .... .... .... 150.1.56.248/30 150.1.56.252/30 150.1.57.0/30 (255.255.255.252) - 64 subnets with 4 addresses per subnet (2 assignable, 1 net id, 1 broadcast) 150.1.57.4/30 150.1.57.8/30 150.1.57.12/30 150.1.57.16/30 .... .... .... 150.1.57.248/30 150.1.57.252/30 150.1.58.0/30 (255.255.255.252) - 64 subnets with 4 addresses per subnet (2 assignable, 1 net id, 1 broadcast) 150.1.58.4/30 150.1.58.8/30 150.1.58.12/30 150.1.58.16/30 .... .... .... 150.1.58.248/30 150.1.58.252/30 150.1.59.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) 150.1.60.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast) ..... ..... ..... 150.1.128.0/27 (255.255.255.224) - 8 subnets with 32 addresses per subnet (30 assignable, 1 net id, 1 broadcast) 150.1.128.32/27 150.1.128.64/27 150.1.128.128/27 150.1.128.160/27 150.1.128.192/27 150.1.128.224/27 ..... ..... ..... 150.1.254.0 (1 subnet, 254 hosts, 1 net id, 1 broadcast) 150.1.255.0 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
Variable Length Subnet Masks... yes... as if subnetting weren't complex enough, then they came along and created this ability to break up a network into multiple subnets with different subnet masks. Your example, while perhaps unnecessarily complex, does appear to correctly capture the essence of VLSM. Let me offer a simpler example, just to confirm the scenario, and in the form you're likely to see the concept tested: Imagine a single Class C network -- 192.168.1.0 -- with 172 computers. We need six subnetworks, two with 50 computers, two with 30 computers, and two with 6 computers. Let's first set up the two 50-computer subnets. For this we'll need two /26 subnets (netmask 255.255.255.192). We'll use the following: 192.168.1.64/26 (192.168.1.65 - 192.168.1.126) 192.168.1.128/26 (192.168.1.129 - 192.168.1.190) Then the two 30 computer subnets. For this we'll need two /27 subnets (netmask 255.255.255.224). We'll use the following: 192.168.1.32/27 (192.168.1.33 - 192.168.1.62) 192.168.1.192/27 (192.168.1.193 - 192.168.1.222) Finally, the two 6-computer subnets. For this we'll need two /29 subnets (netmask 255.255.255.248). We'll use the following: 192.168.1.24/29 (192.168.1.25 - 192.168.1.30) 192.168.1.224/29 (192.168.1.225 - 192.168.1.230) At this point we could also have used several other /29 subnets, including: 192.168.1.8/29 192.168.1.16/29 192.168.1.232/29 192.168.1.240/29 Notice also, although no longer strictly required, I've avoided the use of the first and last subnet in the range (e.g. the subnet with network ID '0' and the subnet with broadcast ID '255'). The end results of our configuration: Net1: 192.168.1.24/29 (6 hosts) Net2: 192.168.1.32/27 (30 hosts) Net3: 192.168.1.64/26 (50 hosts; with 12 unused HostIDs) Net4: 192.168.1.128/26 (50 hosts; with 12 unused HostIDs) Net5: 192.168.1.192/27 (30 hosts) Net6: 192.168.1.224/29 (6 hosts) The key point being here is that we've divided up a specified network into multiple subnets with a *variable* length subnet mask (/26, /27, or /29), depending on the specific subnet. Hope this helps. -- Lawrence Garvin, M.S., MCITP:EA, MCDBA Principal/CTO, Onsite Technology Solutions, Houston, Texas Microsoft MVP - Software Distribution (2005-2009) MS WSUS Website: http://www.microsoft.com/wsus My Websites: http://www.onsitechsolutions.com; http://wsusinfo.onsitechsolutions.com My MVP Profile: http://mvp.support.microsoft.com/profile/Lawrence.Garvin
The end results of our configuration: Thank you for the time to reply...It has helped to solidify my understanding and give me the confidence that I needed. So the ranges: 192.168.1.1 - 192.168.1.23 and 192.168.1.225 - 192.168.1.254 basically go unused until there is a need for them at which point they themselves can be given a VLSM? Thanks again!
