아래의 링크에서 테이블을 찾아보면 서브네트워킹이 무엇인지 확연히
알수 있습니다. 딱 40개가 안나오는 이유도 확실하게 알수 있습니다.
 http://www.ibiblio.org/pub/Linux/docs/HOWTO/mini/IP-Subnetworking   .........
.....
 For our C Class network number of 192.168.1.0, these are some of the
sub-networking options you have:-
 No of   No of
subnets Hosts/net netmask
-----------------------------------------------------------------------
2       126       255.255.255.128 (11111111.11111111.11111111.10000000)
4       62        255.255.255.192 (11111111.11111111.11111111.11000000)
8       30        255.255.255.224 (11111111.11111111.11111111.11100000)
16      14        255.255.255.240 (11111111.11111111.11111111.11110000)
32      6         255.255.255.248 (11111111.11111111.11111111.11111000)
64      2         255.255.255.252 (11111111.11111111.11111111.11111100)
-----------------------------------------------------------------------
  In principle, there is absolutely no reason to follow the above way of
subnetworking where network mask bits are added from the most
significant host bit to the least significant host bit.
However, if you do not do it this way, the resulting IP numbers will
be in a very odd sequence! This makes it extremely difficult for us
humans to decide to which subnetwork an IP number belongs as we are
not too good at thinking in binary (computers on the other hand are
and will use whatever scheme you tell them with equal equanimity).
  Having decided on the appropriate netmask, you then need to work out
what the various Network and broadcast addresses are - and the IP
number range for each of these networks. Again, considering only a
C Class IP Network number and listing only the final (host part)
we have:-
 Netmask  Subnets Network  B'cast   MinIP   MaxIP  Hosts  Total Hosts
--------------------------------------------------------------------
128            2       0     127       1     126    126   
                     128     255     129     254    126     252
 192            4       0      63       1      62     62
                      64     127      65     126     62
                     128     191     129     190     62
                     192     255     193     254     62     248
 224            8       0      31       1      30     30
                      32      63      33      62     30
                      64      95      65      94     30
                      96     127      97     126     30
                     128     159     129     158     30
                     160     191     161     190     30
                     192     223     193     222     30
                     224     255     225     254     30     240
--------------------------------------------------------------------
 : 질문이 있는데요 최대 40개의 호스트를 지원할 수 있도록 네트워크를
: 구현하고자 할 경우 C class에서 서브넷 마스크는 어떻게 줘야 하나요?
: 꼭 좀 답변 주세요~~ 초보의 질문 이었습니다 ^^;