having problems with VSLM

adamchalkley2018

[h=3]Hey guys I am doing the Packet Tracer 9.3.1.4,I am having the problem with VSLM or subnetting[/h]
at the start of the video https://www.youtube.com/watch?v=bfFRiGygHns he explains the IP addresses of the G0/0 and G0/1 interfaces aswell as the PCs on the two subnets

for G0/0 172.31.30.126 and the mask is 255.255.255.128
for G0/1 the ip is 172.31.30.254 with the same subnet mask as above

so why does it start off at 126? why not 172.31.30.1?

and for 6/0/1 why 254? I also thought that it would be 172.31.31.0 since we have subnetted a class B network,which should gives us a /23 value which would give you 2 subnets of 510 hosts each,

so how come we didn't use 31? did we subnet the subnet?

https://seeseenayy.blogspot.ie/2016/12/ccnav3-completed-packet-tracer-9314.html here is a **** sheet it will give you a little peek at the layout

thanks

Neil86

I glanced at the video. Don't let the assigned IP of the router interface confuse you, you can honestly assign the R3 LAN interfaces any address within the subnet (dependent on other things), but it is usually common practice to assign router inerfaces the first IP address. In the video, it is just reversed, he is assigning the last host IP address of the subnet to the router interface and the first host IP address to the PC's (the lab states in the instructions to do so). It is not STARTING the subnet at 126. By using the 255.255.255.128 (/25) subnet mask, the subnets are in increments of 128 (notice positional value of the network bits).

Therefore, the subnets are:

172.31.30.0 - 172.31.30.127 (0-127 = 128 IP address's. Subtract 2 for network ID(.0) and broadcast(.127) = 126 host IP's (.1-.126) This satisfies the host requirements of the lab.

and 172.31.30.128 - 172.31.30.255 (128-255 = 128 IP address's. Subtract 2 for the network ID and broadcast (.128 and .255) for .129-254 = host IP's) This satisfies the host requirements of the lab.

The /23 was the original network, which has now been subnetted. The subnet for the R3 LAN interfaces is clearly a /25 (255.255.255.128 )on the R3 LAN interfaces.

I've had some wine tonight, so I hope I explained this adequately