Block of 8 static ip addresses

Very newbie when it comes to routing and networking, but I am after multiple static ip addresses like I had with Be before they were bought by Sky.

If I obtain a block of 8, I am told only 5 of these are usable, as 3 need to be reserved.

What I want to know is, exactly why 3 need to be reserved, and can i choose which ones to use from the block of 8 or is it automatically assigned?

Thanks

Also, can I assume that in a block of 16, that 13 are usable?

I'm sure there are clearer and more detailed explanations available by searching, but I'll try to explain:

As an example, an 8 address block could be defined as:
192.168.0.0/29

The /29 defines the number of bits of the 32 used by IPv4 that are 'masked' by the network. (the mask can also be expressed as 255.255.255.248). This leaves the last 3 bits usuable for hosts. If you can remember your counting in binary from school, this gives a maximum of 8 permutations - hence the 8 addresses available, which in this case would be:
192.168.0.0
192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.4
192.168.0.5
192.168.0.6
192.168.0.7

For every block defined by a netmask, the first & last (or highest & lowest) addresses have reserved uses.
The first (lowest) address is the network identifier and the last (or highest) is the broadcast address.

So for our example:
192.168.0.0 is the network address.
192.168.0.7 is the broadcast address.

This leaves 6 addresses usable for hosts.
You will need one address assigned to the WAN interface for your router, leaving 5 for other things.

Thanks for the explanation, if I am reading correctly then 2 are reserved (the first and last in the block), but the address for the WAN interface can be any of the others? Any number from 1-6 using your example addresses?

This seems related to a previous thread you started, where my response was this (and I believe has already answered your question(s), so not sure why you are asking it again?):

http://forums.thinkbroadband.com/beunlimited/t/43364...

You don't have to lose the addresses if you use NAT (as I explained). If you are running the IPs as a routable network, 3 will be lost. If you are NATing each public IP to an internal IP, you can configure the router in such a way that ALL IPs are usable.

In reply to a post by nolongerthere:

Thanks for the explanation, if I am reading correctly then 2 are reserved (the first and last in the block), but the address for the WAN interface can be any of the others? Any number from 1-6 using your example addresses?

You might think so, but the ISP will almost always specify which of the remaining 6 is the router address.
They need to know this, as it's where they send the data to.

5 usable public IP addresses are plenty for almost all conceivable uses of a DSL line though - remember that you can use port mapping in a good router (which you'll need if you are going to fully exploit the capabilities) to give you 5 of each possible type of server.
And some of those servers can host more than one site.