1. CS4470 Computer Networking Protocols
7/31/2018
CS Computer Networking Protocols
9. IP addresses
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. TCP/IP Addresses
Addressing in TCP/IP is specified by the Internet Protocol (IP)
Each host is assigned a 32-bit number
Called the IP address or Internet address
Unique across entire Internet
Each IP address is divided into a prefix and a suffix
Prefix identifies network to which computer is attached
No two networks can be assigned the same network number
Suffix identifies computer within that network
No two computers on the same network can have the same suffix, but computers on different networks can have the same suffix
Address format makes routing efficient
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3. Properties of IP Addresses
Network numbers (prefix) are unique
Assignment of network numbers must be coordinated globally to ensure uniqueness
Assigned by ICANN: Internet Corporation of Assigned Names and Numbers
Host addresses (suffix) may be reused on different networks
combination of network number prefix and host address suffix will be unique
assignment of host addresses can be managed locally
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4. Designing IP IP designers chose 32-bit addresses
Still 32 bits in IP version 4 used today
May cause a problem soon, not a large enough address space!
Allocate some bits for prefix, some for suffix
Large prefix, small suffix - many networks, few hosts per network
Small prefix, large suffix - few networks, many hosts per network
Because of variety of technologies, need to allow for both large and small networks
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5. Classful Addressing
Designers chose a compromise - multiple address formats that allow both large and small prefixes
Each format is called an address class
Class of an address is identified by first four bits
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6. Address Classes Class A, B and C are primary classes
Used for ordinary host addressing
Owner of a class is assigned a prefix, gets to pick what machines they want in the suffix
Classes A and B are all allocated!
Class D is used for multicast, a limited form of broadcast
Class E is reserved for future use
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7. Compute the Class of an Address
IP needs to separate the destination address into a prefix and suffix when it handles a packet
Look at first dotted decimal number, and use this table:
Just a little binary number conversion based on first 4 bits
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8. Division of the Address Space
Classing scheme does not yield equal number of networks in each class
Class A:
First bit must be 0
7 remaining bits identify Class A net
27 (= 128) possible class A nets
For the rest:
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9. Classful Addressing Example
Organization with four networks, completely private network
Assign Class A,B,C addresses as appropriate for number of hosts on each network
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10. Limitations of the classful addressing scheme
The IP address space is being exhausted
All networks had to choose one of the three possible sizes, many addresses were wasted.
Ex. A company requests a network address with a block of host address
Has to get a class B address
addresses are unused
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11. Subnet and Classless Addressing
Mechanisms to overcome the limitations
Subnetting and Classless Addressing
Take a single IP network number and allocate the IP addresses with that network number to several physical networks, which are referred to as subnets
The postfix of the IP address is further divided into two parts: subnet id and host address
Each subnet is assigned a subnet number
All host in the same subnet share the same subnet number
The network number and the subnet number now become the new network prefix
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12. Subnet masks
Classless and subnet addressing requires an additional piece of information stored with each address
Known as address masks or subnet masks
Used to specify the boundary between the network prefix and the host prefix
1 bits mark the network prefix
0 bits mark the host portion
Default subnet mask:
Class A:
Class B:
Class C:
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13. Example
We want to share a single class B address among several physical networks (subnets)
We can use a subnet mask of
Top 24 bits are now defined to be the network number
Lower 8 bits are the host number
Since the top 16 bits identify the network in a class B address, there are 3 parts in the IP address: a network part, a subnet part and a host part
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14. Subnetting example
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15. Subnets and Subnet Masks
Each host is configured with both an IP address and a subnet mask for the subnet to which it’s attached
The bitwise AND of the two numbers defines the subnet number of the host and of all other hosts on the same subnet
Eg: At H1, AND =
How will hosts use the subnet mask?
A host: decides whether a destinations address in a packet is local or not
When sending to a destination IP address, perform bitwise AND between the destination IP address and its own subnet mask
If the result equals the subnet number of the sender, then it knows that the destination host is on the same subnet
Otherwise, the packet needs to be sent to a router to be forwarded to another subnet.
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16. Routers and subnets
For simple IP, a router has a forwarding table that consists of entries of the form (networkNumber, NextHop)
To support subnetting, the table now hold entries of the form
Subnet number
Subnet mask
Next hop
Interface 0
Interface 1
Interface 2
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17. Data forwarding algorithm
D = destination IP address
For each forwarding table entry(subnetNumber, subnetMask, nextHop)
D1 = SubnetMask & D
If D1 = subnetNumber, deliver datagram to nextHop (a router or a network)
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18. What’s a subnet ? Another name of physical network
network consisting of 3 subnets
subnet
Another name of physical network
A subnet is a collection of machines that can communicate with each other without the need for routing
Every subnet has at least one router/gateway. If it didn’t, then the subnet couldn’t talk to any machines outside of its network.
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19. Subnets
Recipe
To determine the subnets, detach each interface from its router, creating islands of isolated networks. Each isolated network is a subnet.
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20. Subnets How many subnets? 6! 223.1.2.1 223.1.1.2 223.1.1.1 223.1.1.3
7/31/2018
Subnets
How many subnets? 6!
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21. CIDR Notation CIDR = Classless Inter-Domain Routing
Notation for classless addresses
Specifies the mask associated with an address
Appends a slash to the address with the size of the mask in decimal
Examples:
/16
says 16 bit prefix, 16 bit suffix (Class B)
/28
28 bit prefix, 4 bit suffix
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22. 7/31/2018
CIDR Example 1
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23. Reserved Addresses RFC 1597 – Address Allocation for Private networks
The following blocks are reserved for private networks
Class A
Class B
Class C
We’ll need a gateway to translate from these addresses to the Internet addresses
Network Address Translation
We should never see a machine on the Internet with one of these network prefixes (hopefully)
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24. Special IP Addresses Network Address Directed Broadcast Address
The network itself is assigned an address
So no host can have all zero’s as its IP address suffix
Prefix is the network prefix, suffix is all zero’s
Eg /16
Directed Broadcast Address
Broadcast message to a network
Prefix is the network prefix, suffix is all one’s
So no host can have all one’s as its IP address suffix
Can be routed
Limited Broadcast Address
Broadcast on the local LAN
Entire address is all 1’s, i.e
Won’t be routed
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25. More Special IP Addresses
This Computer Address
To obtain an address automatically when booting, we may use IP to communicate… but we don’t have a correct IP address yet
Use an address of all zero’s to indicate “this computer”
Loopback Address
Any address beginning with 127 indicates the local computer
E.g most common, but could be
Use for testing network applications, no packets leave the computer
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26. Special Address Summary
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