topic why does my subnet have four invalid IP address? in Switches, Hubs, and Modems

why does my subnet have four invalid IP address?

ljshan — Fri, 13 Feb 2009 10:17:06 GMT

I have Procurve 2650 to enable arp proxy. There are two vlans.

The default vlan #1 has network:

192.168.0.0/24 (IP range used: 192.168.0.0~192.168.0.127 and 192.168.0.192~192.168.0.255)

the other vlan #2 is a subnet: 192.168.0.128/26 with IP range: 192.168.0.128~192.168.0.191.

The problem comes from vlan #2. In addition to the 192.168.0.128 as the network and 192.168.0.191 as the broadcast address, i found I can NOT use the 192.168.0.129 and 192.168.0.190 addresses either!

For example, when I set a host in VLAN #2 with IP=192.168.0.129, I can't ping this host from the switch. But when i changed that host's IP to 192.168.0.130, the ping works.

So why can't the subnet 192.168.0.128/26 use the address 192.168.0.129 and 192.168.0.190 as host address?

Re: why does my subnet have four invalid IP address?

Jorge Pinto Leite — Sat, 14 Feb 2009 09:11:19 GMT

If you haven't mistaken the CIDR blocks when writing, both VLAN share the same IP space.
In VLAN#1, 192.168.0.0/24 means that the network address is 192.168.0.0, the broadcast address is 192.168.0.255 and all in between are IP addresses available for hosts.
VLAN#2 uses the same address space, as 192.168.0.[128-191] is included in VLAN#1.
I suppose the switch is becoming confused with that.

Re: why does my subnet have four invalid IP address?

Pieter 't Hart — Mon, 16 Feb 2009 14:35:43 GMT

Jorge points out "both VLAN share the same IP space"
What Jorge means is both subnets overlap!

There is a difference between an ip number range and an ip-subnet.

Although you only use 192.168.0.0 - 192.168.0.127 and 192.168.0.192 -192.168.0.255, the subnet 192.168.0.0/24 spans the whole range from .0 to .255
So the adresses in subnet/vlan2 fall within the same subnet as vlan-1.
You'd better use 192.168.0.0/24 for vlan1 and 192.168.1.0/26 for vlan2 (or 192.168.1.128/26 if you want)
thus the subnets have no overlap