# IP Subnetting

Something you need to know first: Binary Odometer

10.1.1.254 + 1 = 10.1.1.255
10.1.1.255 + 1 = 10.1.2.0
10.1.2.0 + 1 = 10.1.2.1

in reverse:

10.1.2.0 – 1 = 10.1.1.255

## Example 1

172.16.35.123/20 or 172.16.35.123 with the mask 255.255.240.0

### Quick Method

Figure out the subnets:

First subnet = 172.16.32.0

Next subnet = 172.16.48.0

172.16.32.0 + 1 = 172.16.32.1`

Last host = Broadcast – 1
`172.16.47.255 - 1 = 172.16.47.254`

### Subnetting

• Class A subnetting (255.0.0.0) support 1677214 (2^24) host per network, that way too much
• Class B subnetting (255.255.0.0) support 16382 (2^16) host per network, that way too much
• Class C subnetting (255.255.255.0) support 254 (2^8) host, more likely we subnet down to at least 254 hosts or even further

If you subnetting a network only has 2 hosts, you can subnet with (255.255.255.254) or CIDR as /31

### Network, host number

• Networks: 2^(network bits)
• one allocate for the subnet
• one allocate for the broadcast
• Hosts: 2^(host bits) – 2

### Subnetting to be short

1. “stealing” or “taking away” bits from the host portion of an address, and
2. allocating those bits to network portion

## Example 2

Origin network 10.128.192.0/18 need at least 30 subnets as many hosts as possible

1. draw the line with /18 to split network and host
2. 2^5 > 30, need 5 subnet bit, draw the line to split subnet and host
3. network/subnet portion is 8+8+7=23 bits, host portion is 32-23=9 bits
• First subnet: 10.128.192.0/23
• Second subnet: 10.128.194.0/23
• Last subnet: 10.128.254.0/23

###### tags: `ccna`

• layer 3 logical address assigned by an administrator(MAC built in NIC)
• used to identify specific devices on a network
• every device on the internet has a unique IP Address

Street Analogy

• identifies a specific network
• routers route traffic via routing tables, is based on network address(Network ID), not ip address
• identifies a specific endpoint on a network
• we can use a protocal such as ARP to find the host

Ipv4

• connectionless protocal: no sessions formd when transmitted, no status info
• packets treated independently
• may take different paths: load balancing, bandwidth, hopcount
• best effort delivery
• format
• 32 bit with 4 octets
• like DHL or FedEx routing parcel based on an address

### Classes

• Unicast Traffic
• A
• range from 0.0.0.0(00000000) to 127.255.255.255(01111111)
• exceptions:
• 127 is reserved for loopback: 127.0.0.1
• 0 network is reserved for default network: 0.1.1.1
• actual range from 1.0.0.0 to 126.255.255.255
• portions
• first 1 octets: Networks
• last 3 octests: Hosts
• B
• range from 128.0.0.0(10000000) to 191.255.255.255(10111111)
• portions
• first 2 octets: Networks
• last 2 octests: Hosts
• C
• range from 192.0.0.0(11000000) to 223.255.255.255(11011111)
• portions
• first 3 octets: Networks
• last 1 octests: Hosts
• Multicast
• D
• reserved for other purposes
• E

These classes replaced by Classless Inter-Domain Routing(CIDR) in 1993

# OSI Model

• By International Organization of Standard

### Benefits

• Standard and INTEROPERABILITY
• Split development/role: hide developer from lower layer
• Quicker development

### Layers

You need to remember both the name and the layer number

• Layer7: Application
• Layer6: Presentation
• Layer5: Session
• Layer4: Transport
• Layer3: Network