Why can't 0' in the host portion of IP address be used for a host?

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I would guess it is because that is the network address. And, routing protocols use network addresses to determine how to route the network traffic.

docrice

This depends on how large the network is. The last octet can be a 0 and be a host address. Take a /23 for example.

clarson

I think he is talking about an ip address that has so many bits for the network and so many bits for the host. Such as in your /23 example, that is 23 bits for the network and 9 bits for the host. if the host bits are all zero, it is a network address. Doesn't matter if it is a /11 or a /22. if all the host bits are all 0's, then it is a network address. And, routing protocols use the network addresses to route the packets. So, I'm not sure what would happen if you had a 192.168.1.0 /24 host. it seems like the routers would route the packet right. And, a switch uses the mac address. So, that might not be the reason.

OctalDump

If you are using CIDR, then normally it can be (except in the cases where it is invalid or a network address). Classful Addressing, however, can't ever use it for a host address since it is always a network (or invalid) address. So why do networks need their own addresses? Because that's how they made routing work - it makes a lot of sense with classfull addressing.

There are, however, circumstances where modern devices can be confused and not let you use a host address ending in zero, even though it is perfectly legal/valid/acceptable. So some people just avoid them to be on the safe side.

dppagc

all '0' = network address = to identify the subnet
all '1' = broadcast address = for broadcasting