Subnetting made easy by Network Rhinos

1. Subnetting
Breaking one single IP segment to chunks
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2. Subnetting
• Obviously a company wouldn’t put all 16,777,214 hosts into a single logical network, this would be
terrible for performance and security. – For Class A
• They would split their /8 address allocation into smaller subnets and allocate these to different offices
and types of hosts
• For example if they received 12.0.0.0/8, they could allocate the subnet 12.0.1.0/24 to sales
computers in New York, 12.0.2.0/24 to accounting PCs and 12.0.9.0/24 to sales computers in Boston.
Subnetted Apple
Supernet Apple
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3. CIDR – Classless Inter Domain Routing
• A problem with classful addresses was that if a company had more than 254 hosts they would need to
be assigned a Class B network
• They would have much less than the 65,534 (2^16 - 2) hosts allocated, so this wasted a huge amount
of the global address space
• Classless Inter-Domain Routing (CIDR) was introduced in 1993 to alleviate this problem
• CIDR removed the fixed /8, /16 and /24 requirements for the address classes
• For example 175.10.10.0/20 --------- Class B instead of writing /16
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4. Class C /31 Subnet
• Let’s say we’ve been allocated Class C 192.168.1.0/24.
• If we move the line all the way to the right we’re now using /31 (or 255.255.255.254)
• This leaves one bit for the host address, with a possible value of 0 or 1 It borrows 7 bits for
the network address
• This gives us 128 subnets (27) which accommodate 2 hosts each
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
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5. Key Topic : Network and Broadcast address
• Every IP segment should have Network address and Broadcast address.
• How to find it ??
Its actually a simple match to find a Network ID or Network address put all “0” in the host portion.
To find out Broadcast address put all “1” in host portion.
Lets try …
192.168.1.1/24
First 24 bit is Network portion and Next 8 bit is host portion.
If I apply the formula the I will get
Network ID or Network address : 192.168.1.0/24
Broadcast address : 192.168.1.255/24
Quiz :
172.1.2.3/16
192.0.0.1/16
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6. Class C subnet /31
• Let’s say we’ve been allocated Class C 192.168.1.0/24.
• We subnet using /31. Valid host addresses:
• ‒192.168.1.0 to 192.168.1.1
• ‒192.168.1.2 to 192.168.1.3
• ‒Etc., to:
• ‒192.168.1.254 to 192.168.1.255
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
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7. But Wait !!
• Let’s say we’ve been allocated Class C 192.168.1.0/24
• What about the network and broadcast address?!
• /31 breaks the standard rules of IP addressing.
• /31 subnets are supported on Cisco routers for point to point links (which have no need for a network or
broadcast address.)
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8. Class C /30 Subnets
• Let’s say we’ve been allocated Class C 192.168.1.0/24
• Let’s move the line back a place. We’re now using /30 (or 255.255.255.252) This leaves 2 bits for the host
address, 22 = 4, minus 2 for the network and broadcast address = 2 possible hosts
• It borrows 6 bits for the network address
• This gives us 64 subnets (26) which accommodate 2 hosts each
• Valid host addresses:
• ‒ 200.15.10.1 to 200.15.10.2 (network .0, broadcast .3)
• ‒ 200.15.10.5 to 200.15.10.6 (network .4, broadcast .7)
• ‒ Etc., to:
• ‒ 200.15.10.253 to 200.15.10.254 (network .252, broadcast .255)
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
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9. Variable Length Subnet Mask
• Early routing protocols only supported Fixed Length Subnet Masking (FLSM) where all subnets had to be
the same size. Youcouldn’t have a subnet with 14 hosts and another subnet with 64 hosts in the same
network.
Before taking a good meal on Subnettig lets discuss about Private and Public IP
addresses
• Private address are the IP address which cannot go to the internet.
• Public address are routable to the internet.
• Why Private and Public :
• Its simple because what if we exhaust our 4 million IP address.
• Private address can be repeated by multiple enterprise as they will be used to route within the LAN only.
• If a private IP address configured device want to talk to the internet we need NAT – Network Address
Translation. (We will study in future slides)
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10. Subnetting question.
Q) Lets assume your Manager gave you an ip 192.168.1.0/24 and told you that he want 30 hosts instead
of wasting 2^8 – 2 hosts.
Step 1 : Find the Network ID and Broadcast ID
Network ID : 192.168.1.0
Broadcast ID : 192.168.1.254
Step 2 : Write the given IP address in binary format and mark the Network and host portion.
Now we have 8 bits which are host portion and 24 bits for network portion.
However, we need only 32 host, hence lets increase network portion by borrowing some bits from host
portion.
128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
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11. Borrowing bits from host portion
Borrowed Bits New Network Bits = n New Host Bits = h How Many Hosts I can get
2^h - 2
0 24 8 254
1 25 7 126
2 26 6 62
3 27 5 30
4 28 4 14
5 29 3 6
6 30 2 2
7 31 1 0
8 32 0 0
Now we got New Subnet Mask it 255.255.255.224 or Prefix /27
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12. New Subnetted IP’s
• We borrowed 3 bits hence number of networks is 2^3 = 8 (before it was only one network)
Network # Network ID First IP Last IP Broadcast IP
Network 1 192.168.1.0/27 192.168.1.1/27 192.168.1.30/27 192.168.1.31/27
Network 2 192.168.1.0/27 192.168.1.32/27 192.168.1.62/27 192.168.1.63/27
Network 3 192.168.1.0/27 192.168.1.64/27 192.168.1.94/27 192.168.1.95/27
Network 4 192.168.1.0/27 192.168.1.96/27 192.168.1.126/27 192.168.1.127/27
Network 5 192.168.1.0/27 192.168.1.128/27 192.168.1.158/27 192.168.1.159/27
Network 6 192.168.1.0/27 192.168.1.16027 192.168.1.190/27 192.168.1.191/27
Network 7 192.168.1.0/27 192.168.1.192/27 192.168.1.222/27 192.168.1.223/27
Network 8 192.168.1.0/27 192.168.1.224/27 192.168.1.254/27 192.168.1.255/27
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