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danewguy

@vfserver.com

How many are in a "block" of IP addresses?

Exactly what does it mean to acquire a block of static IP addresses? And why is it that how ever many you get, only a portion are "usable"?


tschmidt
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Re: How many are in a "block" of IP addresses?

A block is just that, a block. The size of the block is in powers of two. 2, 4, 8, 16, 32, 64, 128, 256, 512, etc.

The lowest address is reserved for the network address and the highest is used for broadcast. So the usable addresses are two less then the size of the block.

Given the shortage of IPv4 address, IANA just handed out the last /8 blocks to the various registrars, ISPs are going to be very stingy handing out IPv4 address. With IPv6 that is not an issue.

/tom


danewguy

@vfserver.com
Great thanks! Would you be able to provide an example of the 2nd part of your response (usable IPs)?

Also, doesn't NAT allow for virtually unlimited use of a single IP within a LAN?


tschmidt
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join:2000-11-12
Milford, NH
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said by danewguy :

Great thanks! Would you be able to provide an example of the 2nd part of your response (usable IPs)?

Google is your friend. First hit when I searched for "usable ip range"
»www.networkcomputing.com/netdesi ··· 01b.html

said by danewguy :

Also, doesn't NAT allow for virtually unlimited use of a single IP within a LAN?

Yes. NAT, or more correctly Network Address Port Translation (NAPT), allows an unlimited number of addresses to share a single IP address. The number of active sessions is limited by port utilization. TCP and UDP use a 16-bit port value. Use of Port numbers allows a host to be able to manage multiple simultaneous services. Being a 16-bit value means there are 65,535 possible TCP ports and 65,535 UDP ports. A good analogy of ports and addresses is Post Office PO boxes. The IP address is like the street address of the Post Office. Ports are like the internal mail boxes.
»en.wikipedia.org/wiki/NAPT

/tom


danewguy

@vfserver.com
Yikes that's a lot of reading for a dunce like me. I realize everything can be found in Google but often times it's like overkill. Any way to put into layman's terms what the following means?
/30 IP space (1 usable IP)
/29 IP space (5 usable IPs)
/28 IP space (13 usable IPs)
etc. etc.

If you say usable addresses are two less then the size of the block, then why does this provider only have a small fraction of usable IPs?


tschmidt
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join:2000-11-12
Milford, NH
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A /30 subnet means 30 of the 32 bits are used for network address and 2 for host address.

Easier to understand if we represent the 255.255.255.252 subnet mask in binary rather then dotted decimal.

11111111 11111111 11111111 11111100

IP address:
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 - network address
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01 - 1st host address
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10 - 2nd host address
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 - broadcast address

The other subnets work the same way, increasing by power of 2
4,8,16,32,64,128,256,512 etc of which the the lowest and highest addresses are reserved.

I have no idea why the ISP you are referring to is reserving an additional IP address. Need to ask them.

/Tom


rpeAMP

join:2000-12-02
San Antonio, TX
If these are routed blocks, they are reserving an additional address for the gateway address.