IP Addresses and Subnetting

NET 150

Network Addressing

Remember- Most modern network have two types of addresses

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• MAC Addresses:
• Layer 2
• ”burned” into the NIC
• Used by NIC to “pick up” packets addressed to it
• IP Addresses
• Layer 3
• Assigned by Network Admin
• It is the “Street address and Zipcode” that uniquely identifies a host on the network

IP Address

• 32-bit number – just a string of 32 1’s and 0’s

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• Typically displayed as four 8-bit numbers with dots in between
• called dotted decimal notation
• Each 8-bit number is called an octet

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Computer sees:

10000001101010100001001011011100

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We interpret as dotted decimal:

129.170.18.220

IP Place Values

• Learning the first 8 place values in Binary can help

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• The highest value in an octet is 255

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• Looking at the place values, all 1’s in the 8 bits is
• 128+64+32+16+8+4+2+1 = 255
• 35 in Binary:

Network Addressing: IP Address

• IP Address includes both the Network ID (zip code) and Host ID (street address)
• Net ID is always at the beginning of the address
• Tricky bit:
• IP address is always 32 bits but…
• Net ID can be different lengths – anywhere from 8 to 30 bits.
• Host ID is whatever is left over
• Subnet Mask – defines how many bits are in the Net ID

Network Addressing: IP Address (3)

• Example:

Binary IP Address:

10000001101010100001001011011100

Translated to dotted decimal:

129.170.18.220

Subnet mask is /24

Means first 24 bits are Network ID

So,

Network ID is 129.170.18

Host ID is 220

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Subnet Mask

The subnet mask uses 1 and 0 bits to indicate that the corresponding bit of the IP address is either the:

• network (1) or
• the host (0) portion.

IP Address: 172.16.4.35 / 24

Subnet Mask

• Basically, a Subnet Mask indicates how many bits are used for the Network and how many for the Host

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• Orange: Octets contain part of Net ID and Part of Host ID

Subnet Mask

Network Prefixes

• Prefix Mask (aka CIDR Notation):
• The address is followed by a number that represents the number of bits (prefix length), beginning from the left, that apply to the network.
• A slash (/) is used to separate the address and the prefix length.

192.168.10.2/24��Means that the first 24 bits are the network portion.�The last 8 bits are the host portion.

Subnet Mask

IP Address: 10.24.36.2 / 8 Subnet Mask?

IP Address: 10.24.36.2 / 12 Subnet Mask?

IP Address: 10.24.36.2 / 16 Subnet Mask?

IP Address: 10.24.36.2 / 23 Subnet Mask?

IP Address: 10.24.36.2 255.255.224.0 Prefix Mask?

IP Address: 10.24.36.2 255.255.255.192 Prefix Mask?

IP Address: 10.24.36.2 255.255.255.252 Prefix Mask?

IP Address: 10.24.36.2 255.254.0.0 Prefix Mask?

IP Address: 10.24.36.2 255.255.240.0 Prefix Mask?

Subnetting

Organizations are “assigned” a network address to use on the Internet

• aka their zipcode

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• For example, Champlain College:

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• 216.93.144.0/20 (or 255.255.240.0)

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• This means that all Champlain IP’s have to start with the same 20 bits

Champlain Network Assignment

• 216.93.144.0/20 assigned to Champlain

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• Green is NetID, Red is Host ID
• All of our system’s IP addresses have to use the same first 20 bits
• For Hosts – we can use the remaining 12 however we want!

Subnetting

• However, most organizations need more than one “network”

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• As we have learnt, there are limits to how many hosts can be on a network because too many will cause broadcast issues.

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• Therefore, organizations can use some of their host bits to create new networks

Champlain Network Assignment

• Remember: Champlain can’t change first 20,
• but changing mask to /24 can take some more bits from the host to make more networks

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• Green: Original Net ID
• Orange: New bits for Net ID
• Red: Host ID

Champlain Network Assignment

• With /24 (or 255.255.255.0) as the mask, each of these are now different networks
• 216.93.144.0/24
• 216.93.145.0/24
• 216.93.146.0/24
• …
• 216.93.159.0/24

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Rules about IP Addresses

• All 0’s in the Host ID refers to the Network itself – cannot be assigned to a host

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• All 1’s in the Host ID is the broadcast – cannot be assigned to a host

Subnet Table: �How many Host IPs can be assigned

Private and Reserved IP Ranges

Private Ranges not routable on Internet

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/24

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Reserved Ranges

• 127.0.0.0/8 Loopback
• 169.254.0.0/16 Link-Local Host Only
• 224.0.0.0/4 Multicast

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Subnetting Example

Review Subnet Table

Four Networks:

• Central Campus: 1,000 Hosts
• West Campus: 500 Hosts
• East Campus: 500 Hosts
• Wireless Network: 1,600 Hosts

So, Four Networks:

• Central Campus: 1,000 Hosts can use /22 (or 255.255.252.0)
• West Campus: 500 Hosts can use /23 (or 255.255.254.0)
• East Campus: 500 Hosts can use /23 (or 255.255.254.0)
• Wireless Network: 1,600 Hosts can use /21 (or 255.255.248.0)

Example Subnet Assignments

Green: Net Bits

Purple: Subnet Bits

Red: Host bits

Subnet Example