If a computer has an ip address is 10.1.1.5 and sub-net mask is 255.255.255.0 then what does it mean?

It mean your system communticate just10.1.1.0 to10.1.1.255 IP and other than this range not reachable for you system because this is Classless Inter-Domain Routing (CIDR).

without subnetmask IP show as a class A address but when show subnetmask its show class C IP Address.

In simple words:============

• The Network address is 10.1.1.0 and
• The Network can have up to254 IP addresses available to address computers and other network devices in this network
• And ofcourse the computer address on this network is 10.1.1.5 /255.255.255.0

This is a Class A address because it falls in that range where only first8 bits are reserved for network ID, however your subnet mask show that another16 bits are also reserved for the network ID and only the last8 bits are kept for client IDs. 

Changing the subnet mask from8 to24 bits (255.0..0.0 to255.255.255.0) does not change class of any IP.

Subnetting ...

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in1993 to replace the previous addressing architecture of classful network design in the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.[1][2]

Further read here:

http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

besm allah alrahman alrahim

network number is 

and computer (host) number is

without subnetmask IP show as a class A address but when show subnetmask its show class C IP Address.

It means, the given IP address belongs to class A address and it is a private ip address, range of private class a addresses is (10.0.0.0 to10.255.255.255)

u gess the class becuse subnet mask show the class

this mean your ip is belongs to the class a  where as subnet gives you the host which counts the form1 to254 host can occupy in that network its also a private ip . the first three octate are belongs to the network id and last one is host it so that1 to254 host in you network for private ip .

yeah its a classless ip addrees and the subnetmusk255.255.255.0  mean that the first3 portion of the10.1.1.5 ip address is  belongs to the Network part and the4th portion is for the host part even if the10.1.1.5  belongs to the class A ip addresses