# ip prefix-list ( ge 24 le 24 ) ??!

Aug 28th, 2008

Hi Master,

I quite confuse with the values behind the ip prefix list. Can some1 make me understood ?

For example :

ip prefix-list A permit 10.18.0.0/21 ge 24 le 24

Overall Rating: 0 (0 ratings)

## Replies

Giuseppe Larosa Thu, 08/28/2008 - 11:07

Hello William,

the prefix-list means:

permit all prefixes within 10.18.0.0/21 address block if their prefix-length is Greater equal 24 and less equal 24 ==>

all prefixes within 10.18.0.0/21 with prefix length = 24

10.18.1.0/24 is permitted

10.18.2.128/25 is denied

10.18.4.0/23 is denied

if present are permitted:

10.18.0.0/24

10.18.1.0/24

10.18.2.0/24

10.18.3.0/24

10.18.4.0/24

10.18.5.0/24

10.18.6.0/24

10.18.7.0/24

/21 means 3 bit of third byte

Hope to help

Giuseppe

william2u Thu, 08/28/2008 - 18:55

Hi Giuseppe, thanks for ur answer.

some confusion on the /21. Is this mean ? :

128 64 32 16 8 4 2 1

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

0 0 0 0 0 1 1 1

/21 means it allow :

10.18.0.0-10.18.11.0 (Correct me if i'm wrong)

* As for the Greater equal 24 and less equal 24 is only match the mask is it ?

So the answer for this is it only allow 10.18.0.0-10.18.11.0 with the mask of /24 only.

CORRECT ME IF WRONG. thanks !! in learning curve :-)

Giuseppe Larosa Thu, 08/28/2008 - 23:33

Hello William,

when we think of network mask we think of binary digits:

/21 moves 3 bits to the left the net boundary

so with 3 binary digits you can address

000 - 111 but 111 binary = 7 decimal

we use the dotted decimal notation as an help for us human beings but IP addresses are 32 bits numbers.

exercise yourself in binary and only later convert in decimal at each octet / byte boundary.

No problem to be in the learning curve : all of us are in the learning curve

Hope to help

Giuseppe