Advanced IPv4 Addressing Concepts

Question 1

What are the three part structure of an IP address?

Answer 1

network, subnet, host.

Question 2

How is the prefix value /24 calculated?

Answer 2

network bits + subnet bits

Question 3

How to find the mask range?

Answer 3

The range is between: Network bits + Subnet bits(with minimum subnet value) AND Network Bits + Subnet bits(with minimum host bit requirement).

Question 4

What are the three best mask criteria that engineers will choose from?

Answer 4

Highest Subnet bits, highest host bits or the increase both the numbers of supported subnets and hosts(middle).

Question 5

How to find the magic number for which the subnet ID will use?

Answer 5

256-interesting octet number=interval range:
ie. 255.255.224.0 would have magic number of 256-224=32. Subnet ID will be 255.255.32.0, 255.255.64,0, 255.255.96.0 and goes on to 255.255.224.0

Question 6

How to find your broadcast subnet?

Answer 6

When adding the magic number to the network ID so that interesting octet reaches 256, then the previous number is the broadcast subnet. Or the value of the octet(256-magic number).

Question 7

With subnet bits more than 8, we will have an interesting octet and a _______ octet

Answer 7

just-left

Question 8

How to find subnet IDs that has more than 8 subnet bits?

Answer 8

Find the subnet in the fourth octet with the magic number, when it reaches the broadcast number, add 1 to the just-left octet and restart the adding of the magic number until just left reaches 255. Or if having more than 16 bits, add on to the next octet.

Question 9

How do we define VLSM

Answer 9

Variable Length Subnet Mask is defined when a same classful network uses different subnet mask(different prefixes).

Question 10

How to identify overlaps in VLSM?

Answer 10

Identify the subnet ID and subnet broadcast address for the given subnets; compare the range of all and identify the range of addresses that are overlapping.

Question 11

What is the function of summary route?

Answer 11

To minimize memory use of routing table.

Question 12

What is the formula to calculate the best summary route for a given list of subnets?

Answer 12

1. List all subordinate subnets in decimal from low to high along with their broadcast subnet address.
2. Identify the lowest subordinate subnet and the highest subnet broadcast address.
3. identify prefix: pick the shortest prefix length mask of all the subordinate subnet and then subtract 1.
4. Calculate a new potential summary subnet/mask with matching broadcast address based on the lowest subordinate subnet ID from the original list and the current prefix length.
   a. if the calculate range of address includes the entire range from step 2, you have found the best summary route.
   b. If not, subtract 1 from the prefix length used in the most recent calculation and repeat step 4.

Question 13

What is the ACL and its major function?

Answer 13

Access Control Lists has many features, one of them is to filter(deny) or let through(permit) IP packets.

Question 14

What are the four types of IP ACL?

Answer 14

Standard Numbered
Standard Named
Extended Numbered
Extended Named

Question 15

What is first-match logic for all IP ACL types?

Answer 15

When fist logic on the list(going in order) is met, it will stop processing the ACL function list and go with that logic’s outcome(either deny or permit). It will ignore the rest of the list that follows. IF there is no match for any logics on the list, the packet will be denied.

Question 16

How does ACL uses wild card mask?

Answer 16

MC mask uses decimal 0 when router must compare the octet as normal; it uses 255 for router to ignore that octet, considering it already matched.

Question 17

What is the CLI syntax for permitting all address with first single octet 10?

Answer 17

access-list 1 permit 10.0.0.0 0.255.255.255

Question 18

When the Wild Card Mask has 255, what syntax do we use for that octet in IP address?

Answer 18

0.

Question 19

How to find the right wildcard mask to match a subnet?

Answer 19

Use the subnet number as the source value in the access list command; use a wildcard mask found by subtracting the subnet mask from 255.255.255.255:

ie. if 172.16.8.0 255.255.252.0 is the subnet, then the command for the right WC mask would be:
access-list 1 permit 172.16.8.0 0.0.3.255

Question 20

How to configure an ACL to permit all packet? When do we use it?

Answer 20

access-list 1 permit any; use when you want the default setting to permit(default is to deny) all packet after the ACL processed.

Question 21

What is the format of a ACL command in CLI?

Answer 21

access-list (access-list-number) {deny | permit} (source) [source-wildcard mask]

Question 22

What are the steps when implementing a Standard IP ACL

Answer 22

1. Plan the location(router and interface) and direction on that interface. *place near to the destination of the packet. direction should match the packet from source-to-destination direction.
2. Configure one or more access-list command, be mindful of the default deny and first-match logic.
3. Enable the ACL on the chosen router interface, in the correct direction, using the “ip access-group number {in | out} interface subcommand.