Cracking the Coding Interview - Fooo

Solutions to Chapter 17 | Networking
17 4 What is a network / subnet mask? Explain how host A sends a message / packet to
host B when: (a) both are on same network and (b) both are on different networks
Explain which layer makes the routing decision and how
SOLUTION
CareerCup com
pg 84
A mask is a bit pattern used to identify the network/subnet address The IP address consists
of two components: the network address and the host address
The IP addresses are categorized into different classes which are used to identify the network
address
Example: Consider IP address 152 210 011 002 This address belongs to Class B, so:
Network Mask: 11111111.11111111.00000000.00000000
Given Address: 10011000.11010101.00001011.00000010
By ANDing Network Mask and IP Address, we get the following network address:
10011000.11010101.00000000.00000000 (152.210.0.0)
Host address: 00001011.00000010
Similarly, a network administrator can divide any network into sub-networks by using subnet
mask To do this, we further divide the host address into two or more subnets
For example, if the above network is divided into 18 subnets (requiring a minimum of 5 bits
to represent 18 subnets), the first 5 bits will be used to identify the subnet address
Subnet Mask: 11111111 11111111 11111000 00000000 (255 255 248 0)
Given Address: 10011000 11010101 00001011 00000010
So, by ANDing the subnet mask and the given address, we get the following subnet address:
10011000 11010101 00001000 00000000 (152 210 1 0)
How Host A sends a message/packet to Host B:
When both are on same network: the host address bits are used to identify the host within
the network
Both are on different networks: the router uses the network mask to identify the network and
route the packet The host can be identified using the network host address
The network layer is responsible for making routing decisions A routing table is used to
store the path information and the cost involved with that path, while a routing algorithm
uses the routing table to decide the path on which to route the packets
Routing is broadly classified into Static and Dynamic Routing based on whether the table is
fixed or it changes based on the current network condition
2 5 4