cisco networking academy chabot college elec 99.05 subnet masking
TRANSCRIPT
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Chabot CollegeChabot College
ELEC 99.05ELEC 99.05Subnet Masking
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Logical Address CompositionLogical Address Composition
Though the exact length and format of a logical (layer 3) address differs depending on the protocol, all logical addresses share this basic formula:
NETWORK NUMBER
NETWORK NUMBER HOST NUMBERHOST NUMBER
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Logical Address CompositionLogical Address Composition
IP address may have an additional field, the subnet field, that is used to identify smaller networks within a larger network.
NETWORK NUMBER
NETWORK NUMBER
SUBNET NUMBERSUBNET NUMBER
HOST NUMBER
HOST NUMBER
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Address ClassesAddress Classes
Class A (0 - 127)
Class B (128-191)
Class C (192-223)
Network Host Host Host
Network Network Host Host
Network Network Network Host
1st octet 2nd octet 3rd octet 4th octet
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Address ClassesAddress Classes
Class A
Class B
Class C
85 45 31 158
168 65 114 201
210 144 235 56
Network Host
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
• The solution to the IP address shortage was thought to be the subnet mask.
• Formalized in 1985, the subnet mask breaks a single class A, B or C network in to smaller pieces.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
SubnettingSubnetting
What’s happened to the host fields?
Network Network Subnet Host
8 bits 8 bits
8 bits are now used to represent subnets. Only 8 bits remain for possible hosts.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
SubnettingSubnetting
What’s happened to the host fields?
Network Network Subnet Host
8 bits 8 bits
8-bit subnet field = 28 subnets = 256 subnets. 8-bit host field = 28 hosts = 256 hosts. Remember, we started with 65,536 hosts!
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
• The Subnet Mask is a 32-bit number.• Its job is to tell routers (and humans)
which bits are network number and which bits are used to represent hosts.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
• The Subnet Mask corresponds to the IP address.
• A “1” bit in the subnet mask means that the corresponding bit in the IP address should be read as a network number
• A “0” bit in the subnet mask means that the corresponding bit in the IP address should be read as a host bit.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 00000000 00000000
10101001 11000111 01000101 10001001Address
Mask
Here, the first 16 bits of the mask are set to “1.” Thus, the first 16 bits (2 octets) of the IP address are network number.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 00000000 00000000
10101001 11000111 01000101 10001001Address
Mask
Network Network Host Host
The mask shows that the first two octets refer to the network number.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 00000000 00000000
10101001 11000111 01000101 10001001Address
Mask
Network Network Host Host
The mask shows that the first two octets refer to the network number.
255 255 0 0
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 11111111 00000000
10101001 11000111 01000101 10001001Address
Mask
Network Network Network Host
Here, the first 24 bits are set to “1” in the subnet mask. Thus, the first 24 bits (3 octets) of the IP address are network number.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 11111111 00000000
10101001 11000111 01000101 10001001Address
Mask
Network Network Network Host
Here, the first 24 bits are set to “1” in the subnet mask. Thus, the first 24 bits (3 octets) of the IP address are network number.
255 255 255 0
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 11111111 11100000
10101001 11000111 01000101 10001001Address
Mask
Network Network Network Host
Here, the first 27 bits of the subnet mask are set to “1.” Thus, the first 27 bits of the IP address refer to network number.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11111111 11111111 11111111 11100000
10101001 11000111 01000101 10001001Address
Mask
Network Network Network Host
Here, the first 27 bits of the subnet mask are set to “1.” Thus, the first 27 bits of the IP address refer to network number.
255 255 255 224
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Default Subnet MasksThe Default Subnet Masks
Class A or /8
Class B or /16
Class C or /24
255 0 0 0
255 255 0 0
255 255 255 0
If any additional bits in the subnet mask are set to “1,” then subnets have been created.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
255 255 255 0
207 21 54 0Address
Mask
Network Network Network Host
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
The Subnet MaskThe Subnet Mask
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 00000000
255 255 255 0
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 1-bit Mask (/25)Class C : 1-bit Mask (/25)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 10000000
255 255 255 128
Masks can be written different ways. When the phrase “1-bit” mask is used, that means 1 more bit than the default. This example can also be called a 25-bit mask, or /25 (there are 25 network bits).
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 2-bit Mask (/26)Class C: 2-bit Mask (/26)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11000000
255 255 255 192
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 3-bit Mask (/27)Class C: 3-bit Mask (/27)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11100000
255 255 255 224
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 4-bit Mask (/28)Class C: 4-bit Mask (/28)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11110000
255 255 255 240
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 5-bit Mask (/29)Class C: 5-bit Mask (/29)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11111000
255 255 255 248
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 6-bit Mask (/30)Class C: 6-bit Mask (/30)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11111100
255 255 255 252
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 7-bit Mask (/31)Class C: 7-bit Mask (/31)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11111110
255 255 255 254
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C: 8-bit Mask (/32)Class C: 8-bit Mask (/32)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network
11111111 11111111 11111111 11111111
255 255 255 255
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 1-bit Mask (/25)Class C : 1-bit Mask (/25)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 10000000
255 255 255 128
Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)
00000000
0Address
Mask
Host
10000000
128
How many subnet possibilities with 1 bit?
21 = 2 different possibilities
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)
000000000 =
128 =
255 =
The last octet can be any value from 0 - 255.
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)
000000000 =
128 =
255 =
How many hosts can be on each subnet?
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
27 = 128 different possibilities MINUS TWO.
7 bits
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Why “Minus Two”?Why “Minus Two”?• Two special host addresses are “reserved”:
– the address of the subnet itself(all zeros)
– the broadcast address for the subnet(all ones)
000000000 =
128 =
255 =
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
Subnet addr.
Broadcast addr.
Subnet addr.
Broadcast addr.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Network #, Broadcast AddressNetwork #, Broadcast Address
000000000 =
All zeros in the HOST portion of the address is reserved for network (or subnet) number.
All ones is reserved for the network (or subnet) BROADCAST address.
000000011 =
Decimal Binary
0010010036 =
01111111127 =
10000000128 =
11111111255 =
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
So, which subnet is this host on?
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
207.21.54.35
255.255.255.128Subnet #0
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
Are these two hosts on the same subnet?
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
207.21.54.115
255.255.255.128
No.207.21.54.129
255.255.255.128
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
Unfortunately, older TCP/IP software cannot handle subnets with all zeros in the subnet field.
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
More bad news, older TCP/IP software cannot handle subnets with all ones in the subnet field.
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
For now, using subnets that have all zeros or all ones in the subnet field should be considered illegal. (It’s not really, but….)
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
128 =
255 =
Bottom line, a 1-bit mask results in no usable addresses.
01111111127 =
10000000
11111111
Subnet #0
Subnet #1
Decimal Binary
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 2-bit Mask (/26)Class C : 2-bit Mask (/26)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11000000
255 255 255 192
Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 2-bit Mask (/26)Class C : 2-bit Mask (/26)
00000000
0Address
Mask
Host
11000000
192
How many subnet possibilities with 2 bits?
22 = 4 different possibilities:
00 01 10 11
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
64 =
127 =
0011111163 =
01000000
01111111
Subnet #0
Subnet #1
Decimal Binary
128 =
191 =
10000000
10111111
192 =
255 =
11000000
11111111
Subnet #2
Subnet #3
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
64 =
128 =
How many hosts can be on each subnet?
0011111163 =
01000000
01111111
Subnet #0
Subnet #1
Decimal Binary
26 = 64 different possibilities MINUS TWO.
6 bits
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Which subnets can be used?Which subnets can be used?
000000000 =
64 =
127 =
0011111163 =
01000000
01111111
Subnet #0
Subnet #1
Decimal Binary
128 =
191 =
10000000
10111111
192 =
255 =
11000000
11111111
Subnet #2
Subnet #3
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Putting it togetherPutting it togetherAssuming the mask: 255.255.255.192, which addresses are legal for hosts?
1) 207.21.54.35
2) 207.21.54.63
3) 207.21.54.65
4) 207.21.54.190
5) 207.21.54.195
000000000 =
64 =
127 =
0011111163 =
01000000
01111111
Subnet #0
Subnet #1
Decimal Binary
128 =
191 =
10000000
10111111
192 =
255 =
11000000
11111111
Subnet #2
Subnet #3
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 3-bit Mask (/27)Class C : 3-bit Mask (/27)
11001111 00010101 00110110 00000000
207 21 54 0Address
Mask
Network Network Network Host
11111111 11111111 11111111 11100000
255 255 255 224
Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Class C : 3-bit Mask (/27)Class C : 3-bit Mask (/27)
00000000
0Address
Mask
Host
11100000
224
How many subnet possibilities with 3 bits?
23 = 8 different possibilities:
000 001 010 011 100 101 110 111
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
32 =
63 =
0001111131 =
00100000
00111111
Decimal Binary
64 =
95 =
01000000
01011111
96 =
127 =
01100000
01111111
10000000128 =
160 =
191 =
10011111159 =
10100000
10111111
Decimal Binary
192 =
223 =
11000000
11011111
224 =
255 =
11100000
11111111
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Last OctetLast Octet
000000000 =
32 =
64 =
How many hosts can be on each subnet?
0001111131 =
00100000
00111111
Subnet #0
Subnet #1
Decimal Binary
25 = 32 different possibilities MINUS TWO.
5 bits
CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY
Which subnets can be used?Which subnets can be used?
000000000 =
32 =
63 =
0001111131 =
00100000
00111111
Decimal Binary
64 =
95 =
01000000
01011111
96 =
127 =
01100000
01111111
10000000128 =
160 =
191 =
10011111159 =
10100000
10111111
Decimal Binary
192 =
223 =
11000000
11011111
224 =
255 =
11100000
11111111