[SunRescue] Networking details (was Re: OT: Advice on Certification)
Greg A. Woods
rescue at sunhelp.org
Tue May 8 12:47:20 CDT 2001
[ On Tuesday, May 8, 2001 at 18:24:58 (+0200), Magnus Hultin wrote: ]
> Subject: Re: [SunRescue] Networking details (was Re: OT: Advice on Certification)
>
> On Tue, 8 May 2001, Bill Bradford wrote:
>
> > I've *never* seen "Class C" described as ONE SPECIFIC netblock...
> >
> > In common usage, everything I've run into -
> >
> > "Class C" = a /24 netblock,
> > 254 usable IP addresses (x.x.x.1 to x.x.x.254, with .0 being the
> > network and .255 the broadcast), 255.255.255.0 netmask..
>
> When I studied this about two years ago, my teachers told
> the story like this:
>
> A long time ago, when IP numbers were a new thing, you could
> be assigned either a class A block, a class B block or a class
> C block of numbers.
>
> Class A would be a /8 network block in the range 1.0.0.0 -
> 126.255.255.255, with 10.x.x.x reserved for private use.
>
> Class B would be a /16 block in the range 128.0.0.0 -
> 191.255.255.255, with 172.16.0.0 - 172.31.255.255 reserved
> for private use.
>
> Class C would be a /24 block in the range 192.0.0.0 -
> 255.255.255.255 with 192.168.0.0 - 192.168.255.255
> reserved.
Yes, that's mostly right. The definition of a Class C address is more
correctly:
Class C would be a /24 block in the range 192.0.0.0 -
223.255.255.255 with 192.168.0.0 - 192.168.255.255
reserved.
and of course strictly speaking if you're talking about network numbers
then you should show the ranges without the hosts as in 192.0.0.0 -
223.255.255.0.
Also of course there's the /8 netblock 127.0.0.0 - 127.255.255.255 which
is reserved for "loopbacks" and which should never appear on any
network. There are some other wee spots where addresses can no longer
be assigned from too, the complete list being (last updated about a year
ago, so check ARIN et al to clarify anything):
0.0.0.0/8 IANA RESERVED-1
1.0.0.0/8 IANA RESERVED-9
2.0.0.0/8 IANA RESERVED-2
5.0.0.0/8 IANA RESERVED-5
10.0.0.0/8 RFC 1918 (IANA RESERVED-10)
14.0.0.0/8 IANA NET-PDN
23.0.0.0/8 IANA RESERVED-23 (NET-DDN-TC-NET)
27.0.0.0/8 IANA RESERVED-27
31.0.0.0/8 IANA RESERVED-12
37.0.0.0/8 IANA RESERVED-37
39.0.0.0/8 IANA RESERVED-39A
41.0.0.0/8 IANA RESERVED-41A
42.0.0.0/8 IANA RESERVED-42
58.0.0.0/8 IANA RESERVED-59
59.0.0.0/8 IANA RESERVED-59
60.0.0.0/8 IANA RESERVED-60
65.0.0.0-95.255.255.255 IANA RESERVED-7
96.0.0.0-126.255.255.255 IANA RESERVED-8
127.0.0.0/8 LOOPBACK
128.0.0.0/16 IANA RESERVED-3
128.66.0.0/8 IANA NET-TEST-B
169.254.0.0/16 IANA LINKLOCAL (DHCP client auto-config space)
172.16.0.0/12 RFC 1918 (IANA-BBLK-RESERVED)
191.255.0.0/16 IANA RESERVED-4
192.0.0.0/16 IANA NET-ROOT-NS-LAB
192.0.0.192/32 illegal printservices discovery
192.0.1.0/24 IANA NET-ROOT-NS-LIVE
192.0.2.0/24 IANA NET-TEST
192.168.0.0/16 RFC 1918 (IANA-CBLK-RESERVED)
197.0.0.0/8 IANA RESERVED-13
201.0.0.0/8 IANA RESERVED-14
223.255.255.0/24 IANA RESERVED-5
240.0.0.0/7.255.255.255 Class E (RESERVED)
In essence the first zero bit within the first four bits of the address
identifies the "Class" of an address. If you look at the bit masks
you'll see the following (which is similarly represented in many
references on TCP/IP, in this case more or less from p.64 of Volume 1 of
"Internetworking with TCP/IP Principles: Protocols, and Architecture"
4th Edition, Douglas E. Comer):
bits -> note the jumps ->
+-------------------------------------------------------------+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |16 |24 31|
+-------------------------------------------------------------+
Class A: | 0 | network-number | host-number |
+-------------------------------------------------------------+
Class B: | 1 | 0 | network-number | host-number |
+-------------------------------------------------------------+
Class C: | 1 | 1 | 0 | | host-number |
+-------------------------------------------------------------+
Class D: | 1 | 1 | 1 | 0 | multicast-address |
+-------------------------------------------------------------+
Class E: | 1 | 1 | 1 | 1 | reserved |
+-------------------------------------------------------------+
Sub-net and super-net concepts were an intermediate extension that
helped guide the way towards full class-less addressing using a string
of any length of all-ones bits as the network number mask (well, of
course a /31 is more or less useless and the ranges above
223.255.255.255 are still reserved for multicast and other
as-yet-unspecified uses).
--
Greg A. Woods
+1 416 218-0098 VE3TCP <gwoods at acm.org> <woods at robohack.ca>
Planix, Inc. <woods at planix.com>; Secrets of the Weird <woods at weird.com>
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