CCNA: Subnet masking

Subnetting an IP Network is done primarily for better utilization of available IP address space, and routing purpose. Other reasons include better organization, use of different physical media (such as Ethernet, WAN, etc.),  and securing network resources. A subnet mask enables you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. A logical AND operation between the IP address and the subnet mask provides the Network Address. For example, using our test IP address and the default Class C subnet mask, we get: 192.189.210.078: 1100 0000.1011 1101.1101 0010.0100 1110 Class C IP Address 255.255.255.000: 1111 1111.1111 1111.1111 1111.0000 0000 Default Class C subnet mask 192.189.210.0         1100 0000 1011 1101 1101 0010 0000 0000

As can be seen above, by using and AND operator, we can compute the network portion of an IP address.  The network portion for the IP address given in the above example is 192.189.210.0, and the host portion of the IP address is 078.
Given below is a table that provides binary equivalent of decimal values.  For binary conversion, take first octet of a given IP address (in dotted decimal form), and lookup the binary value. Then take the second octet and lookup the binary value, and so on.

Binary Conversion Table

Decimal	Binary	Decimal	Binary	Decimal	Binary	Decimal	Binary
0	0000 0000	64	0100 0000	128	1000 0000	192	1100 0000
1	0000 0001	65	0100 0001	129	1000 0001	193	1100 0001
2	0000 0010	66	0100 0010	130	1000 0010	194	1100 0010
3	0000 0011	67	0100 0011	131	1000 0011	195	1100 0011
4	0000 0100	68	0100 0100	132	1000 0100	196	1100 0100
5	0000 0101	69	0100 0101	133	1000 0101	197	1100 0101
6	0000 0110	70	0100 0110	134	1000 0110	198	1100 0110
7	0000 0111	71	0100 0111	135	1000 0111	199	1100 0111
8	0000 1000	72	0100 1000	136	1000 1000	200	1100 1000
9	0000 1001	73	0100 1001	137	1000 1001	201	1100 1001
10	0000 1010	74	0100 1010	138	1000 1010	202	1100 1010
11	0000 1011	75	0100 1011	139	1000 1011	203	1100 1011
12	0000 1100	76	0100 1100	140	1000 1100	204	1100 1100
13	0000 1101	77	0100 1101	141	1000 1101	205	1100 1101
14	0000 1110	78	0100 1110	142	1000 1110	206	1100 1110
15	0000 1111	79	0100 1111	143	1000 1111	207	1100 1111
16	0001 0000	80	0101 0000	144	1001 0000	208	1101 0000
17	0001 0001	81	0101 0001	145	1001 0001	209	1101 0001
18	0001 0010	82	0101 0010	146	1001 0010	210	1101 0010
19	0001 0011	83	0101 0011	147	1001 0011	211	1101 0011
20	0001 0100	84	0101 0100	148	1001 0100	212	1101 0100
21	0001 0101	85	0101 0101	149	1001 0101	213	1101 0101
22	0001 0110	86	0101 0110	150	1001 0110	214	1101 0110
23	0001 0111	87	0101 0111	151	1001 0111	215	1101 0111
24	0001 1000	88	0101 1000	152	1001 1000	216	1101 1000
25	0001 1001	89	0101 1001	153	1001 1001	217	1101 1001
26	0001 1010	90	0101 1010	154	1001 1010	218	1101 1010
27	0001 1011	91	0101 1011	155	1001 1011	219	1101 1011
28	0001 1100	92	0101 1100	156	1001 1100	220	1101 1100
29	0001 1101	93	0101 1101	157	1001 1101	221	1101 1101
30	0001 1110	94	0101 1110	158	1001 1110	222	1101 1110
31	0001 1111	95	0101 1111	159	1001 1111	223	1101 1111
32	0010 0000	96	0110 0000	160	1010 0000	224	1110 0000
33	0010 0001	97	0110 0001	161	1010 0001	225	1110 0001
34	0010 0010	98	0110 0010	162	1010 0010	226	1110 0010
35	0010 0011	99	0110 0011	163	1010 0011	227	1110 0011
36	0010 0100	100	0110 0100	164	1010 0100	228	1110 0100
37	0010 0101	101	0110 0101	165	1010 0101	229	1110 0101
38	0010 0110	102	0110 0110	166	1010 0110	230	1110 0110
39	0010 0111	103	0110 0111	167	1010 0111	231	1110 0111
40	0010 1000	104	0110 1000	168	1010 1000	232	1110 1000
41	0010 1001	105	0110 1001	169	1010 1001	233	1110 1001
42	0010 1010	106	0110 1010	170	1010 1010	234	1110 1010
43	0010 1011	107	0110 1011	171	1010 1011	235	1110 1011
44	0010 1100	108	0110 1100	172	1010 1100	236	1110 1100
45	0010 1101	109	0010 1101	173	1010 1101	237	1010 1101
46	0010 1110	110	0110 1110	174	1010 1110	238	1110 1110
47	0010 1111	111	0110 1111	175	1010 1111	239	1110 1111
48	0011 0000	112	0111 0000	176	1011 0000	240	1111 0000
49	0011 0001	113	0111 0001	177	1011 0001	241	1111 0001
50	0011 0010	114	0111 0010	178	1011 0010	242	1111 0010
51	0011 0011	115	0111 0011	179	1011 0011	243	1111 0011
52	0011 0100	116	0111 0100	180	1011 0100	244	1111 0100
53	0011 0101	117	0111 0101	181	1011 0101	245	1111 0101
54	0011 0110	118	0111 0110	182	1011 0110	246	1111 0110
55	0011 0111	119	0111 0111	183	1011 0111	247	1111 0111
56	0011 1000	120	0111 1000	184	1011 1000	248	1111 1000
57	0011 1001	121	0111 1001	185	1011 1001	249	1111 1001
58	0011 1010	122	0111 1010	186	1011 1010	250	1111 1010
59	0011 1011	123	0111 1011	187	1011 1011	251	1111 1011
60	0011 1100	124	0111 1100	188	1011 1100	252	1111 1100
61	0011 1101	125	0111 1101	189	1011 1101	253	1111 1101
62	0011 1110	126	0111 1110	190	1011 1110	254	1111 1110
63	0011 1111	127	0111 1111	191	1011 1111	255	1111 1111

Example Question: Which of the following is a Class C IP address?

A. 10.10.14.118       B. 135.23.112.57       C. 191.200.199.199       D. 204.67.118.54

                          

Correct Answer: D.

Explanation:
IP addresses are written using decimal numbers separated by decimal points. This is called dotted decimal notation of expressing IP addresses. The different classes of IP addresses is as below:

Class	Format	Leading Bit  pattern	Network address Range	Maximum  networks	Maximum hosts
A	N.H.H.H	0	0-126	127	16,777,214
B	N.N.H.H	10	128-191	16,384	65,534
C	N.N.N.H	110	192-223	2,097,152	254

Network address of all zeros means "This network or segment".
Network address of all 1s means " all networks", same as hexadecimal of all Fs.
Network number 127 is reserved for loopback tests.
Host (Node) address of all zeros mean "This Host (Node)".
Host (Node) address of all 1s mean "all Hosts (Nodes) " on the specified network.