Re: LAN IP addresses
- From: ibuprofin@xxxxxxxxxxxxxxxxxxxxxx (Moe Trin)
- Date: Sat, 06 Sep 2008 19:54:40 -0500
On Sat, 06 Sep 2008, in the Usenet newsgroup comp.os.linux.networking, in
article <6if0vbFpbhs9U1@xxxxxxxxxxxxxxxxxx>, Felix Tiede wrote:
IANA (http://www.iana.org) says there are 256 subnets with a maximum
of 256-2 (one network base and one broadcast address) addresses each
in the192.168.x.y address space. Bigger subnets should use
172.16-31.x.y which allows 16 subnets with a maximum of (256^2)-2 =
65534 addresses each. Or, if even larger subnets are required, use
10.x.y.z which allows one huge subnet of (256^3)-2 = 16777214
addresses.
Actually, RFC1918 states
3. Private Address Space
The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
We will refer to the first block as "24-bit block", the second as
"20-bit block", and to the third as "16-bit" block. Note that (in
pre-CIDR notation) the first block is nothing but a single class A
network number, while the second block is a set of 16 contiguous
class B network numbers, and third block is a set of 256 contiguous
class C network numbers.
An enterprise that decides to use IP addresses out of the address
space defined in this document can do so without any coordination
with IANA or an Internet registry.
It _MENTIONS_ pre-CIDR Class-ful sizes for those who missed the word
that RFC1517 replaced the class-ful notation two years earlier. What
that document does NOT do is specify what network mask to use.
By the way, there is nothing sacred about the "zero" address, and some
operating systems specifically do allow use of the "network" address
as a _host_ address.
This is a relict of the old times when there were no netmasks, but
it is still valid...
Why look at page 4 of RFC0923 and think what kind of network setup
had to be used for the first few networks listed there. Bear in mind
that Ethernet had a physical limit of 4100 meters between the furthest
hosts, and an unrealistic maximum of 62250 systems on a single
collision domain (DIX document XNSS-018211). So if they could only
have 62250 hosts on a network, how did they use a /16 (or a "Class B"
as it was called then) - never mind a /8 or "Class A"? Did they only
use 62250 addresses? Any idea how big Yuma Proving Grounds is?[1]
You may want to read RFC0950 - it predates RFC1918 by ten years, and
RFC1517 (CIDR) by eight.
0950 Internet Standard Subnetting Procedure. J.C. Mogul, J. Postel.
Aug-01-1985. (Format: TXT=37985 bytes) (Updates RFC0792) (Also
STD0005) (Status: STANDARD)
So if you intend to widen up your netmask you should also use
appropriate address space.
Is the block large enough to contain your networks? 192.168.0.0/16 is
large enough to hold a single 255.255.0.0 - and that is all that counts.
Look again at RFC0791. No one cares how you arrange your address block.
If you want to create 16384 /30 subnets within a /16, or use it as a
single /16 (or anything in between), go ahead and do so. NO ONE CARES
how it looks "inside", as that is _your_ problem, not the world's.
Old guy
[1] about 1200 square miles, or 4200 square kilometers
.
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