Cidr ?

[ScottJ]

Can someone help me figure out a simple way of how to determine what supernet blocks are contained within a particular CIDR notation?

Here is RFC 1878 for reference:

Table

The following table lists the variable length subnets from 1 to 32,
the CIDR [3] representation form (/xx) and the Decmial equivalents.
(M = Million, K=Thousand, A,B,C= traditional class values)

Mask value: # of
Hex CIDR Decimal addresses Classfull
80.00.00.00 /1 128.0.0.0 2048 M 128 A
C0.00.00.00 /2 192.0.0.0 1024 M 64 A
E0.00.00.00 /3 224.0.0.0 512 M 32 A
F0.00.00.00 /4 240.0.0.0 256 M 16 A
F8.00.00.00 /5 248.0.0.0 128 M 8 A
FC.00.00.00 /6 252.0.0.0 64 M 4 A
FE.00.00.00 /7 254.0.0.0 32 M 2 A
FF.00.00.00 /8 255.0.0.0 16 M 1 A
FF.80.00.00 /9 255.128.0.0 8 M 128 B
FF.C0.00.00 /10 255.192.0.0 4 M 64 B
FF.E0.00.00 /11 255.224.0.0 2 M 32 B
FF.F0.00.00 /12 255.240.0.0 1024 K 16 B
FF.F8.00.00 /13 255.248.0.0 512 K 8 B
FF.FC.00.00 /14 255.252.0.0 256 K 4 B
FF.FE.00.00 /15 255.254.0.0 128 K 2 B
FF.FF.00.00 /16 255.255.0.0 64 K 1 B
FF.FF.80.00 /17 255.255.128.0 32 K 128 C
FF.FF.C0.00 /18 255.255.192.0 16 K 64 C
FF.FF.E0.00 /19 255.255.224.0 8 K 32 C
FF.FF.F0.00 /20 255.255.240.0 4 K 16 C
FF.FF.F8.00 /21 255.255.248.0 2 K 8 C
FF.FF.FC.00 /22 255.255.252.0 1 K 4 C
FF.FF.FE.00 /23 255.255.254.0 512 2 C
FF.FF.FF.00 /24 255.255.255.0 256 1 C
FF.FF.FF.80 /25 255.255.255.128 128 1/2 C
FF.FF.FF.C0 /26 255.255.255.192 64 1/4 C
FF.FF.FF.E0 /27 255.255.255.224 32 1/8 C
FF.FF.FF.F0 /28 255.255.255.240 16 1/16 C
FF.FF.FF.F8 /29 255.255.255.248 8 1/32 C
FF.FF.FF.FC /30 255.255.255.252 4 1/64 C
FF.FF.FF.FE /31 255.255.255.254 2 1/128 C
FF.FF.FF.FF /32 255.255.255.255 This is a single host route

Sample Question:

q) List he /23's in CIDR block 204.24.104.0 /21

---

I'm trying to get a better grasp on this to help troubleshoot routing trouble from our customers to certian networks. and sometimes I have a hard time determining what /n a particualr destination host is contained within (so I can obviously see where we're learning the routes from in BGP peering).

Thx for any assist all ;)


--Scott

[AbecX]

Easiest way is to just get an ip calculator :)

[ScottJ]

will those tell you each of the supernets within a larger /n? Looking into this now...

[ScottJ]

ok think i'm getting a grip on this:


204.24.104.0 /21 -OR- 204.24.104 /21

that means we'll examine the 5 highorder bits of 104.

104(dec) 01101000(bin)

5 high order bits = 01101

/21 has 8 C networks, the ones that match 01101 are:

104 - 111 decimal, 204.24.104.0 -> 204.24.111.255

Thats 2048 addresses

What are the /23 networks in this /21??????

/23 = 2 C nets, so there will be 4 /23's in this /21 (2x4=8)


204.24.104 /23 = 2C's
204.24.106 /23 = 2C's
204.24.108 /23 = 2C's
204.24.110 /23 = 2C's

----------------------------

204.24.104 /21 = 8C's


These are the 4 /23 networks contained within 204.24.104 /21


Does that look in line to you all??????