CISCO NETWORKING ACADEMY Chabot College ELEC Subnet Masking

CISCO NETWORKING ACADEMY Logical 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 HOST NUMBER

CISCO NETWORKING ACADEMY Logical 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 SUBNET NUMBER HOST NUMBER

CISCO NETWORKING ACADEMY Address Classes Class A ( ) Class B ( ) Class C ( ) NetworkHost Network Host Network Host 1st octet2nd octet3rd octet4th octet

CISCO NETWORKING ACADEMY Address Classes Class A Class B Class C NetworkHost

CISCO NETWORKING ACADEMY The 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.1985

CISCO NETWORKING ACADEMY Subnetting What’s happened to the host fields? Network SubnetHost 8 bits 8 bits are now used to represent subnets. Only 8 bits remain for possible hosts.

CISCO NETWORKING ACADEMY Subnetting What’s happened to the host fields? Network SubnetHost 8 bits 8-bit subnet field = 2 8 subnets = 256 subnets. 8-bit host field = 2 8 hosts = 256 hosts. Remember, we started with 65,536 hosts!

CISCO NETWORKING ACADEMY The 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 ACADEMY The 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 ACADEMY The Subnet Mask Address 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 ACADEMY The Subnet Mask Address Mask Network Host The mask shows that the first two octets refer to the network number.

CISCO NETWORKING ACADEMY The Subnet Mask Address Mask Network Host The mask shows that the first two octets refer to the network number

CISCO NETWORKING ACADEMY The Subnet Mask Address Mask 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 ACADEMY The Subnet Mask Address Mask 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 ACADEMY The Subnet Mask Address Mask 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 ACADEMY The Subnet Mask Address Mask 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 ACADEMY The Default Subnet Masks Class A or /8 Class B or /16 Class C or / If any additional bits in the subnet mask are set to “1,” then subnets have been created.

CISCO NETWORKING ACADEMY The Subnet Mask Address Mask Network Host

CISCO NETWORKING ACADEMY The Subnet Mask Address Mask Network Host

CISCO NETWORKING ACADEMY Class C : 1-bit Mask (/25) Address Mask Network Host 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 ACADEMY Class C: 2-bit Mask (/26) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 3-bit Mask (/27) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 4-bit Mask (/28) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 5-bit Mask (/29) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 6-bit Mask (/30) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 7-bit Mask (/31) Address Mask Network Host

CISCO NETWORKING ACADEMY Class C: 8-bit Mask (/32) Address Mask Network

CISCO NETWORKING ACADEMY Class C : 1-bit Mask (/25) Address Mask Network Host 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 ACADEMY Last Octet 1-bit Mask (/25) Address Mask Host How many subnet possibilities with 1 bit? 2 1 = 2 different possibilities

CISCO NETWORKING ACADEMY Last Octet 1-bit Mask (/25) = 128 = 255 = The last octet can be any value from = Subnet #0Subnet #1 DecimalBinary

CISCO NETWORKING ACADEMY Last Octet 1-bit Mask (/25) = 128 = 255 = How many hosts can be on each subnet? = Subnet #0Subnet #1 DecimalBinary 2 7 = 128 different possibilities MINUS TWO. 7 bits

CISCO NETWORKING ACADEMY 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) = 128 = 255 = = Subnet #0Subnet #1 DecimalBinary Subnet addr. Broadcast addr. Subnet addr. Broadcast addr.

CISCO NETWORKING ACADEMY Network #, Broadcast Address = 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 = DecimalBinary = = = =

CISCO NETWORKING ACADEMY Last Octet = 128 = 255 = So, which subnet is this host on? = Subnet #0Subnet #1 DecimalBinary Subnet #0

CISCO NETWORKING ACADEMY Last Octet = 128 = 255 = Are these two hosts on the same subnet? = Subnet #0Subnet #1 DecimalBinary No

CISCO NETWORKING ACADEMY Last Octet = 128 = 255 = Unfortunately, older TCP/IP software cannot handle subnets with all zeros in the subnet field = Subnet #0Subnet #1 DecimalBinary

CISCO NETWORKING ACADEMY Last Octet = 128 = 255 = More bad news, older TCP/IP software cannot handle subnets with all ones in the subnet field = Subnet #0Subnet #1 DecimalBinary

CISCO NETWORKING ACADEMY Last Octet = 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….) = Subnet #0Subnet #1 DecimalBinary

CISCO NETWORKING ACADEMY Last Octet = 128 = 255 = Bottom line, a 1-bit mask results in no usable addresses = Subnet #0Subnet #1 DecimalBinary

CISCO NETWORKING ACADEMY Class C : 2-bit Mask (/26) Address Mask Network Host 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 ACADEMY Class C : 2-bit Mask (/26) Address Mask Host How many subnet possibilities with 2 bits? 2 2 = 4 different possibilities:

CISCO NETWORKING ACADEMY Last Octet = 64 = 127 = = Subnet #0Subnet #1 DecimalBinary 128 = 191 = = 255 = Subnet #2Subnet #3

CISCO NETWORKING ACADEMY Last Octet = 64 = 128 = How many hosts can be on each subnet? = Subnet #0Subnet #1 DecimalBinary 2 6 = 64 different possibilities MINUS TWO. 6 bits

CISCO NETWORKING ACADEMY Which subnets can be used? = 64 = 127 = = Subnet #0Subnet #1 DecimalBinary 128 = 191 = = 255 = Subnet #2Subnet #3

CISCO NETWORKING ACADEMY Putting it together Assuming the mask: , which addresses are legal for hosts? 1) ) ) ) )

CISCO NETWORKING ACADEMY Class C : 3-bit Mask (/27) Address Mask Network Host 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 ACADEMY Class C : 3-bit Mask (/27) Address Mask Host How many subnet possibilities with 3 bits? 2 3 = 8 different possibilities:

CISCO NETWORKING ACADEMY Last Octet = 32 = 63 = = DecimalBinary 64 = 95 = = 127 = = 160 = 191 = = DecimalBinary 192 = 223 = = 255 =

CISCO NETWORKING ACADEMY Last Octet = 32 = 64 = How many hosts can be on each subnet? = Subnet #0Subnet #1 DecimalBinary 2 5 = 32 different possibilities MINUS TWO. 5 bits

CISCO NETWORKING ACADEMY Which subnets can be used? = 32 = 63 = = DecimalBinary 64 = 95 = = 127 = = 160 = 191 = = DecimalBinary 192 = 223 = = 255 =