Re: Netmasks for dummies

From: Centurion (spam.this_at_nowhere.tld)
Date: 02/22/05 

Date: Tue, 22 Feb 2005 20:18:01 +1100

StupidScript wrote:

> Just so I understand the system, or at least get a crib sheet for it,
> please let me know if the following is incorrect:
>
> ============= Scenario 1 - 8 addresses: =============
> My ISP has assigned me IPs "198.113.64.1" through "198.113.64.8"
>
> The netmask which accomodates this could be written as either:
> "255.255.255.248" or as "/29"

198.113.64.1 and 198.113.64.8 are NOT valid network boundaries for a /29
subnet. 198.113.64.0-198.113.64.7 is a valid IP block for a /29 subnet.

/29 or 255.255.255.248 defines the subnet mask which assigns a certain
number of bits to the network and the remainder to the host.

/29 has 29 bits allocated to the network and 3 bits allocated to the hosts.
Therefore 2^3=8 which is the number of valid addresses on a /29 subnet. Of
that 8, 1 is a network address and 1 is a broadcast address.

In your case (example):
198.113.64.0 = network
198.113.64.1-198.113.64.6 = hosts/machines etc.
198.113.64.7 = broadcast

192.113.64.8 = the next /29 network's address.... and so on.

> In my iptables, I could DROP all packets coming from any machine on my
> subnet ONLY by identifying the incoming IP range as:
> "198.113.64.1/255.255.255.248" or as "198.113.64.1/29"

replace .1 with .0 and you've got it.

> ============= Scenario 2 - 16 addresses: =============
> My ISP has assigned me IPs "198.113.64.1" through "198.113.64.16"
>
> The netmask which accomodates this could be written as either:
> "255.255.255.240" or as "/28"

Gah - see above. 198...0 - 198...15 is the address range. .0 is the network
address, .15 is the broadcast, .1-.14 are valid host addresses.

> In my iptables, I could DROP all packets coming from any machine on my
> subnet ONLY by identifying the incoming IP range as:
> "198.113.64.1/255.255.255.240" or as "198.113.64.1/28"

replace .1 with .0

> ============= Last Scenario - 128 addresses: =============
> My ISP has assigned me IPs "198.113.64.1" through "198.113.64.128"

Godammit :P (See above).

> The netmask which accomodates this could be written as either:
> "255.255.255.128" or as "/25"
>
> In my iptables, I could DROP all packets coming from any machine on my
> subnet ONLY by identifying the incoming IP range as:
> "198.113.64.1/255.255.255.128" or as "198.113.64.1/25"

s/\.1/\.0/
  
> Am I off base? I appreciate any responses.

On EVERY IP network, 2 addresses are 'wasted' because every IP networks
requires a network address and a broadcast address. Therefore the smallest
subnet possible is a /30 (255.255.255.252) which leaves 30 allocated to the
network and 2 bits to the address giving 4 addresses in total; 1 network, 2
hosts and 1 broadcast. I use these for connecting routers together on uor
internal network. That way the only valid addresses are each end of the
link ;)

Get hold of "ipsc" - IP Subnet Calculator. It's a command line utility
that's been compiled for nearly every OS known to man (and woman) AFAIK.

James 

-- 
Your lucky number has been disconnected.

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