topic Re: broadcast address in Operating System - HP-UX

broadcast address

Grant Wenstrand — Wed, 16 Aug 2000 15:42:02 GMT

Hello all,

Can someone please explain to me how HPUX determines a broadcast address? I am not an expert in networking, but I thought the last octet of a broadcast address was always ".255". What am I missing here?

Re: broadcast address

Antoanetta Naghiu — Wed, 16 Aug 2000 15:47:30 GMT

Take a look into one of the following:

http://www.docs.hp.com/dynaweb/hpux11/@Generic__CollectionView

http://docs.hp.com/hpux/communications/#doc
or HP_UX Sys Admin -part II - course material

Re: broadcast address

CHRIS_ANORUO — Wed, 16 Aug 2000 15:47:31 GMT

HPUX uses the last octet also. check your /etc/rc.config.d/netconf. You can also use lanadmin to check the errors and broadcast packets.

Re: broadcast address

Kofi ARTHIABAH — Wed, 16 Aug 2000 15:53:45 GMT

The simple answer is No, it is not always .255... the long answer is:

depending on the "class" of your network, your broadcast is not necessarily .255
these days with CIDR, networks can be broken on boundries other than the traditional class A, B, or C
- a class C network is called a /24 network because the first 24 bits of the IP address forms the network address - in that case, the last 8 = 32 - 24 bits are set to all 1's or all 0's for the broadcast address (ie. 0 or 255). The netmask for this is 255.255.255.0

In otherwords, your netmask determines your broadcast address. Your broadcast is obtained by ORing the network (IP) address with (wo's compliment of your netmask - ie. 0.0.0.255 in the case of a class C/24.

Re: broadcast address

Grant Wenstrand — Wed, 16 Aug 2000 16:33:51 GMT

Thank you all for your input, but I am still not getting it.

Perhaps I should be more specific. If I assign the following:
IP address: 206.36.231.130
netmask: 255.255.255.224
Why do I get a broadcast address of:
206.36.231.159

Re: broadcast address

Kofi ARTHIABAH — Wed, 16 Aug 2000 16:47:08 GMT

OK IP address =....130 = 10000010 (binary)
netmask = ...224= 11100000 (binary)
compliment netmask = 00011111 (binary)
so:

IP Addr OR netmask compliment = 10000010
OR = 00011111
= 10011111 = 159

that is why your broadcast is 159.

You do not have a full class C network - you have only a /27 network which is a part of the class C (/24) network. In the scheme you have, there will be another network that starts from .160