CISCO NETWORKING ACADEMY Chabot College ELEC Subnet Review

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

CISCO NETWORKING ACADEMY IP 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 The Subnet Mask Address Mask Network Host Mask has: 1s in the network/subnet portion 0s in the host portion

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 Octet - binary/decimal equivalents (memorize) Bit

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.

CISCO NETWORKING ACADEMY All Zeros / All Ones The address of the subnet itself (the wire) has a host portion that is all zeros. The broadcast address for the subnet has a host portion that is all ones = 128 = 255 = = Subnet #0Subnet #1 DecimalBinary Subnet addr. Broadcast addr. Subnet addr. Broadcast addr.

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 subnets can we have with 1 bit? 2 1 = 2 different possibilities 1 bit

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 Test Question Type 1 IP Address Subnet Mask How many subnets are created by this mask? How many hosts can be on each subnet? Steps: Identify address class & network bits Count subnet bits; express as power of 2 (-2) Count host bits; express as power of 2 (-2)

CISCO NETWORKING ACADEMY Step 1: Identify Class & Network Bits Address Mask Network SubnetHost CLASS B - ignore network octets 16 bits

CISCO NETWORKING ACADEMY Step 2: Count Subnet Bits Address Mask Network SubnetHost bits 2 10 = 1,024 possible subnets MINUS TWO

CISCO NETWORKING ACADEMY Step 3: Count Host Bits Address Mask Network SubnetHost bits 2 6 = 64 possible hosts / subnet MINUS TWO.

CISCO NETWORKING ACADEMY Test Question Type 2 Long Method IP Address Subnet Mask What is this host’s subnetwork number? What is the broadcast address for the subnet that contains this host? Method: Count host bits; determine how many host addresses/subnet List the host ranges in each subnet (Multiples of 2 N ) Find the subnet containing the target address The first address will have all 0s in the host portion The last address will have all 1s in the host portion

CISCO NETWORKING ACADEMY Step 1: Count Host Bits Address Mask Network Host bits 2 6 = 64 host address / subnet Increment host ranges by 64.

CISCO NETWORKING ACADEMY Remaning Steps: List host ranges in each subnet Find subnet containing target address First address is subnet, last is broadcast Subnet address is: Subnet broadcast address is:

CISCO NETWORKING ACADEMY IP Address Subnet Mask What is this host’s subnetwork number? Steps: 1. Convert address & mask to binary 2. Replace host portion of address with all 0s 3. Convert each octet back to dotted decimal Tip: Ignore any octets with a mask that is all 1s Test Question Type 2 Quick Method

CISCO NETWORKING ACADEMY Tip: Ignore octets masked with all 1s: Address Mask The values of these octets are the same for the subnet network & broadcast address.

CISCO NETWORKING ACADEMY Step 1: Convert to binary Address Mask Convert the remaining octet(s).

CISCO NETWORKING ACADEMY Step 2: Set host portion of address to all 0s Address Mask Host (Value of this octet will change)

CISCO NETWORKING ACADEMY Step 3: Convert octet back to dotted decimal Address Mask Host Subnet address is:

CISCO NETWORKING ACADEMY Quick Method - Broadcast addr. IP Address Subnet Mask What is the broadcast address for the subnet that contains this host? Same method, but use all 1s in step 2: 1. Convert address & mask to binary 2. Set host portion of address to all 1s 3. Convert each octet back to dotted decimal

CISCO NETWORKING ACADEMY Tip: Ignore octets masked with all 1s: Address Mask

CISCO NETWORKING ACADEMY Step 1: Convert to binary Address Mask

CISCO NETWORKING ACADEMY Step 2: Set host portion of address to all 1s Address Mask Host (Value of this octet will change)

CISCO NETWORKING ACADEMY Step 3: Convert each octet back to dotted decimal Address Mask Host Subnet broadcast address is:

CISCO NETWORKING ACADEMY Second Example, Quick Method IP Address Subnet Mask What is this host’s subnet number? What is the broadcast address for that subnet? Steps: Convert address & mask to binary Replace host portion of address with all 0s for subnet Replace host portion of address with all 1s for broadcast Convert each octet back to dotted decimal

CISCO NETWORKING ACADEMY Ignore octets masked with all 1s: Address Mask

CISCO NETWORKING ACADEMY Step 1: Convert to binary Address Mask

CISCO NETWORKING ACADEMY Subnet Address Step 2: Set host portion of address to all 0s Address Mask Host (Value of these octets will change) Subnet

CISCO NETWORKING ACADEMY Address Mask Host Step 3: Convert each octet back to dotted decimal 640 Subnet address is: Subnet

CISCO NETWORKING ACADEMY Broadcast Address Step 2: Set host portion of address to all 1s Address Mask Host (Value of these octets will change) Subnet

CISCO NETWORKING ACADEMY Address Mask Host Step 3: Convert each octet back to dotted decimal Subnet broadcast address is: Subnet

CISCO NETWORKING ACADEMY Network Subnet Mask What is the fourth usable host address on the second usable subnet? Steps: 1. Ignore octets masked with all ones. 2. Replace host portion of address with binary four 3. Replace subnet portion with binary two 3. Convert each octet back to dotted decimal Test Question Type 3 Quick Method

CISCO NETWORKING ACADEMY Step 1: Ignore octets masked with all 1s: Address Mask These octets won’t change.

CISCO NETWORKING ACADEMY Step 2: Set host portion to binary four Address Mask Host Reserved Fourth usable host First usable host

CISCO NETWORKING ACADEMY Step 3: Set subnet portion to binary two Address Mask Host Reserved Second usable subnet First usable subnet

CISCO NETWORKING ACADEMY Step 3: Convert octet back to dotted decimal Address Mask Host Fourth usable address in second usable subnet is:

CISCO NETWORKING ACADEMY 1. Ignore octets masked Convert remaining octets to binary. 2. Segment binary address into portions: network | subnet | host 4. Use your understanding of binary addressing to answer the problem. 5. Convert each octet back to dotted decimal. Quick Method - General Strategy