Gregory K. Ruiz-Ade wrote:
On Feb 22, 2007, at 4:13 AM, Ralph Shumaker wrote:
Then why would one use /27:
(same host)
12.169.40.36
00001100 10101001 00101000 00100100 - 12.169.40.36 - host
11111111 11111111 11111111 11100000 - /27 netmask (if I got it)
----------------------------------- - bitwise AND the two together
00001100 10101001 00101000 00100000 - 12.169.40.32 ???
Since the last 5 bits will always AND to zeros, does this mean that
12.169.40.0 through 12.169.40.31 all belong to 12.169.40.36 /27?
The whole point of CIDR (Classless Internet Domain Routing, i think)
is to be able to divvy out portions of network space smaller than a
Class C.
Running your example through a nifty tool called 'ipcalc' (which has
seen multiple variations, but this one is a perl script I got off
sourceforge a couple years ago):
[EMAIL PROTECTED](ttyp1):~ 23 % ipcalc 12.169.40.36 27
Address: 12.169.40.36
Address: 00001100.10101001.00101000.001 00100
Netmask: 255.255.255.224 == 27
Netmask: 11111111.11111111.11111111.111 00000
=>
Network: 12.169.40.32/27 (Class A)
Network: 00001100.10101001.00101000.001 00000
Broadcast: 12.169.40.63
Broadcast: 00001100.10101001.00101000.001 11111
HostMin: 12.169.40.33
HostMin: 00001100.10101001.00101000.001 00001
HostMax: 12.169.40.62
HostMax: 00001100.10101001.00101000.001 11110
Hosts: 30
So, your math was a bit off. But, a /27 is a 32-address span. In
any network, though, you lose three addresses right off the top: The
lowest address (in this case, .32) is the network address. The
highest (.63) is the broadcast address.
The third address is whichever one you assign to be the gateway/
router for the subnet. Convention strongly suggests either the
lowest or highest useable host address, either .33 or .62 in this case.
The smallest subnet you can have is a /30, with 4 addresses: network,
two hosts, and broadcast. Given that one of your addresses has to be
a router, you end up with only a single usable host IP address. So,
for one host, you end up carving out 4 IP addresses. This is why
most Cable Internet providers simply give you DHCP addresses for your
hosts on a very large subnet. For example, my router at home (Cox
Internet):
[EMAIL PROTECTED](ttyp2):~ 23 % ipcalc 68.111.245.13 22
Address: 68.111.245.13
Address: 01000100.01101111.111101 01.00001101
Netmask: 255.255.252.0 == 22
Netmask: 11111111.11111111.111111 00.00000000
=>
Network: 68.111.244.0/22 (Class A)
Network: 01000100.01101111.111101 00.00000000
Broadcast: 68.111.247.255
Broadcast: 01000100.01101111.111101 11.11111111
HostMin: 68.111.244.1
HostMin: 01000100.01101111.111101 00.00000001
HostMax: 68.111.247.254
HostMax: 01000100.01101111.111101 11.11111110
Hosts: 1022
Cox uses a /22 for our network node, and, to my recollection, has a 1
IP per house policy, with additional IPs being an additional cost
item on the bill. To preserve as many IP addresses as possible, they
lump the whole neighborhood into that /22 instead of giving each home
a /30 for a colossal savings in IP addresses.
How networks are subnetted is primarily an implementation issue for
the network provider.
Now, did I actually succeed in answering your question, or just throw
a lot of information at you? :)
Maybe (either way), but had you run "ipcalc 12.169.40.39 27" or "ipcalc
12.169.40.54 27", the result would still be a range from 32 through 63?
If that's the case, then my math wasn't off, just my understanding as to
what my resulting 12.169.40.32 meant.
If this is so (and if I'm understanding correctly), then the resulting
range would start at that address (inclusive) and slurp up 2^(32-27) ==
2^5 == 32 addresses. So 12.169.40.36 just indicates one of the
addresses within the range, 27 indicates what kind of range (bitwise
subtracted from 2^32) and is bitwise ANDed to the given address to find
the starting address. The first address is always the network address
(host unusable), the last address is always the broadcast (host
unusable), and in the resulting pool either the first or the last
address *should* be used for the gateway (host unusable). Right?
If Network address, Broadcast address, and Gateway address are
unassignable to hosts because of dedication to their respective tasks,
could you summarize those tasks? Network? Broadcast? Gateway? Thanks
in advance.
(I think I'm understanding this much more than before.)
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