Last IP in subnet?

Hi,

Just wondering if im going mad or not (usually the case).
Could somebody please tell me if im wrong and if so explain why?

Q:
Configure Router1's serial interface with the last ip address in subnet 192.168.168.184 /30

My Answer:
192.168.168.187

Softwares Answer:
192.168.168.186

My Workings:
192.168.168.184
255.255.255.252

So goes up in intervals of 4
......192.168.168.180
192.168.168.184 - 192.168.168.188
x.x.x.184 equals network address
x.x.x.188 equals broadcast address
last usable address x.x.x.187

Any help is appreciated as always.

Thanks
Nick

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Comments

mikej412

surfthegecko wrote: »

So goes up in intervals of 4
......192.168.168.180
192.168.168.184 - 192.168.168.188
x.x.x.184 equals network address

If you meant that "192.168.168.184 - 192.168.168.188" as a range, that's 5 addresses.

192.168.168.188 is the network address of the next subnet

192.168.168.184 - network address
192.168.168.185 - host address
192.168.168.186 - host address
192.168.168.187 - broadcast address

surfthegecko

doh!!!!

My workings out were:

x.x.x.164
x.x.x.168
x.x.x.172
x.x.x.176
x.x.x.180
x.x.x.184
x.x.x.188

Then I did x.x.x.184 - 188
Which gave me my x.x.x.187 as broadcast etc etc
Which is where I slipped up.

Thanks Mike

mikej412

surfthegecko wrote: »

doh!!!!

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amp2030

As a quick check, sort of a CRC to your subnetting :P, keep in mind that the last octet in

* the subnet number - always even (easy to remember)
* the first valid ip - always odd (subnet # + 1 so odd)
* the broadcast ip - always odd (subnet # - 1 so even)
* the last valid ip - always even (broadcast # - 1 so odd)

miller811

amp2030 wrote: »

As a quick check, sort of a CRC to your subnetting :P, keep in mind that the last octet in

* the subnet number - always even (easy to remember)
* the first valid ip - always odd (subnet # + 1 so odd)
* the broadcast ip - always odd (subnet # - 1 so even)
* the last valid ip - always even (broadcast # - 1 so odd)

I never realized that.
Thanks for the great tip.

Megadeth4168

amp2030 wrote: »

As a quick check, sort of a CRC to your subnetting :P, keep in mind that the last octet in

* the subnet number - always even (easy to remember)
* the first valid ip - always odd (subnet # + 1 so odd)
* the broadcast ip - always odd (subnet # - 1 so even)
* the last valid ip - always even (broadcast # - 1 so odd)

Hehe... I never really thought of it that way! Awesome post.

surfthegecko

yeah like that little factual tid-bit.
Thanks

tim100

surfthegecko wrote: »

Hi,

Just wondering if im going mad or not (usually the case).
Could somebody please tell me if im wrong and if so explain why?

Q:
Configure Router1's serial interface with the last ip address in subnet 192.168.168.184 /30

My Answer:
192.168.168.187

Softwares Answer:
192.168.168.186

My Workings:
192.168.168.184
255.255.255.252

So goes up in intervals of 4
......192.168.168.180
192.168.168.184 - 192.168.168.188
x.x.x.184 equals network address
x.x.x.188 equals broadcast address
last usable address x.x.x.187

Any help is appreciated as always.

Thanks
Nick

If your network address is 192.168.168.184 /30 that leaves 2 bits.
just do 2 to the power of 2 and subtract 2 for the broadcast and network address which leaves 2 usable host IP addresses - 185 and 186.
187 is your broadcast.

Kaminsky

surfthegecko wrote: »

doh!!!!

My workings out were:

x.x.x.164
x.x.x.168
x.x.x.172
x.x.x.176
x.x.x.180
x.x.x.184
x.x.x.188

Then I did x.x.x.184 - 188
Which gave me my x.x.x.187 as broadcast etc etc
Which is where I slipped up.

Thanks Mike

Couldn't you just kick yourself ! I remember that feeling very, very well from subnetting.

Very quick math though.
Work out the 4 width of the subnet.
184 is divisable by 4
188 is the next one (as you painfully aware)
take off bcast (188 -1) and then -2 and there is the last IP.
With much larger subnets, a good speed trick is to work from the next subnet and go backwards rather than count up from the subnet start (184, +1, +2, +3 =187).