IP Adresses

[Bill]

On IDNet's BB Pricing page, the prices for extra addresses are shown as:

2 Usable Addresses /30
£ 4.26/month (£5.00 incl. VAT)
6 Usable Addresses /29
£ 8.51/month (£10.00 incl. VAT)

etc.

What do the /29 and /30 numbers mean?

[Simon_idnet]

Hi Bill

It refers to the number of IP addresses required. It is in CIDR notation (Classless Inter-domain Routing): a /30 is 4 IP addresses (the first is always reserved for the netblock and the last is always reserved for the broadcast address (within the netblock)) giving 2 usable addresses; a /29 is 8 addresses giving 6 usable.

For some light reading (esp good for insomniacs) see here: http://www.ripe.net/info/info-services/addressing.html

Regards
Simon

[Bill]

Thanks Simon, especially for the link- I'm pretty sure I understand it now 

If I've got it right- the number after the slash is the number of significant "1"s in the mask, 2 raised to the power of the number of zeroes represents the total number of addresses available, subtract 2 to get the usable total.

There must be an easier way!

[MoHux]

Thanks Simon, especially for the link- I'm pretty sure I understand it now 

If I've got it right- the number after the slash is the number of significant "1"s in the mask, 2 raised to the power of the number of zeroes represents the total number of addresses available, subtract 2 to get the usable total.

There must be an easier way!

thinks .............. wonder if there's an internet version of the plain english society?? 

[Bill]

 

[Adam]

It's not all that hard!

For example (this should be fun ):

192.168.10.17/30 means the first 30bits is the network prefix, which leaves 2bits for host addresses (IPv4 addresses are 32bits, 8bits for each section). Now working out the possible combinations of the 2bits means there can be four possible combinations (00, 01, 10, 11); the first is used as the network address and the last is the broadcast address, which leaves 2 usable addresses.

Of course it would be easier to just work out..

2 ^ 2 = 4 available - 2 = 2 usable (/30)
2 ^ 3 = 8 available - 2 = 6 usable (/29)
2 ^ 4 = 16 available -2 = 14 usable (/28)
2 ^ 5 = 32 available -2 = 30 usable (/27)

(Yes.. the pattern does continue all the way to /1 )



Adam

[Scott]

Splotch....!

Yes...that was the sound of my cerebellum imploding ....wqer...43q 34%
Urk