Subnetworking zero CIDR notation

Question

A friend and I are arguing about the CIDR notation.

What happend if a host have a /0 mask ?

My friend said he always be able to communicate with all computers in the world and the host will always ask to its router to route its packets.

But I think the host will never communicate with its router so its packets can't be routed.

An other question : Is it correct to write IP address in hexadecimal ?

(Sry for my possible English mistakes)

Answer 1

if a host has a /0 subnetmask, it will never communicate with the default gateway because it will always asume the target is within its own LAN range. So further communication will be done with the MAC address via ARP resolvment.

so if:

• a target ip is within the subnet => MAC and ARP
• a target ip isn't within the subnet => send to default gateway

Answer 2

By 'host have /0 mask' do you mean the default route at the host has a /0 mask? In that case, my answer below. Going back to the basics: a /0 mask would mean 0.0.0.0 in dotted decimal notation. This means that in absence of any better match, it would forward all packets to this interface. If a router is connected to that interface, it will then send it further. For complete logic you can refer : http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml