Subnets and Subnet Masks in Networks

1. Subnets and Subnet Masks CS 447 Networks and Data Communication Department of Computer Science Southern Illinois University Edwardsville Fall, 2019 Dr. Hiroshi Fujinoki E-mail: hfujino@siue.edu

2. CS 447 Networks and Data Communication IP Subnet and IP Subnet Mask Subnet_Mask/001

3. Why do we need “Subnet Mask” Internet CS 447 Networks and Data Communication internal router  How are “subnet addresses” used?  Who use “subnet addresses”? forwards incoming packets using subnet address subnet 1 Public Network (Internet) gateway router subnet 2 32-bit IP address forwards incoming packets using subnet address forwards incoming packets using subnet address Host Address Domain Address Host Address Domain Address subnet 4 subnet 3 this IP packet is delivered to the destination host using the “host address” a network domain (e.g. “146.163.x.x” for “SIUE domain”) Subnet Address Subnet_Mask/002

4. Why do we need “Subnet Mask” CS 447 Networks and Data Communication • IP has a way to specify the number of hosts within a domain (CIDR-prefix), but does not have a way to specify the number of subnets in a domain • Since each domain has a different number of sub-networks, there must be a mechanism to let routers know which bits in each IP address indicate a subnet address (as long as your network does not have any sub-network, you do not have to manually set up “subnet mask” – or “default subnet mask” is automatically applied, which indicates “0 subnet in this domain”) Subnet_Mask/003

5. If there are two subnetworks within a domain  If there are four subnetworks within a domain  10010000 10100011 XXXXXXXX XXXXXXXX If there are 2n subnetworks within a domain  146 163 XX XX We need to indicate how many bits are assigned for subnetworks CS 447 Networks and Data Communication Owners of a network domain can create as many subnets as they want. Problem “manually ” (= “the network administrator needs to set up the routers in the domain”) “subnet mask” 1 bit to indicate subnetwork 2 bits to indicate subnetwork n bits to indicate subnetwork Example: a domain of “146.163.X.X/16” Subnet_Mask/004

6. 146 163 XX XX 10010000 10100011 XXXXXXXX XXXXXXXX Example We know that the last 16 bits indicate the host addresses (since /16 assumes to use the last 16 bits for host address) CS 447 Networks and Data Communication IP Subnet and IP Subnet Mask Purpose: To indicate which portion of an IP address indicates host address SIUE IP Network Domain: 146.163.XX.XX If we do NOT have any subnetwork Subnet_Mask/005

7. Mask for subnetwork Mask for domain address Subnet Mask: Invert 1 and 0s 10010000 10100011 01101101 00000001 00000000 00000000 00001111 11111111 00000000 00000000 00001101 00000001 11111111 11111111 11110000 00000000 13 1 CS 447 Networks and Data Communication How to use Subnet Mask #1: Get Host Address • If SIUE has 16 subnetworks • Try to figure out the host address of 146.163.109.1 (255.255.240.0) BINARY AND 146.163.109.1: Subnet_Mask/006

8. Mask for domain address Subnet Indicator Subnet Mask: BINARY AND 146.163.109.1: 10010000 10100011 01101101 00000001 10010000 10100011 01100000 00000000 11111111 11111111 11110000 00000000 IP domain address Subnetwork address CS 447 Networks and Data Communication How to use Subnet Mask #2: Get Domain Address and Subnet Address • If SIUE has 16 subnetworks • Try to figure out the domain and subnetwork address of 146.163.109.1 Subnet_Mask/007