Protocol identifies the protocol carried inside the

• Protocol : Identifies the protocol carried inside the
next header after this IPv4 header – typically TCP or
UDP.
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IP Header Fields
• Header Checksum : Allows error checking of IP packets
• Source Address : 4-byte IPv4 source address for this
packet
• Destination Address : 4-byte IPv4 destination address for
this packet
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Ask Google for one?
How do you get an IP address
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Do you want a
static or dynamic
IP address
It depends!

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DHCP
• Network admin can set static or dynamic address.
• For dynamic, device will broadcast to
Dynamic Host Configuration Protocol server
• DHCP server will return the following values
required to send IPv4 data :
o IP Address
o Subnet Mask
o Default gateway address
o DNS server address
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DHCP Server
• DHCP server maintains pool of free IP addresses for
each subnet and allocates with a lease time .
• Device must renew IP address before lease time is up
or the DHCP server will re-claim it.
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Example
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What is a router!
Network traffic goes through
routers…
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What’s a Router?
• A router is a device with multiple network interfaces.
o Ethernet interfaces connect the router to Ethernet switches
o Serial interfaces connect routers to other routers over point‐
to‐point links
• Each interface has its own IP address and subnet mask
• Each interface connects to an IP subnet of devices
o For us: subnet = Ethernet LAN or a point‐to‐point link
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A Router’s job
Simply, forward packets from one IP subnet to
another IP subnet:
• It receives packets on one interface
• It looks up packet’s IP destination address in routing table
• It retransmits the packet out another interface to get it closer to its
destination.
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With different tasks
Different types of routers!
Types of Routers
• Wireless Home Routers combine a Wi-Fi access point, an
Ethernet switch and a simple access router into a single box for
home networking and Internet access across a DSL or Cable
Modem connection.
• Enterprise routers interconnect Ethernets within an business
organization.
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Types of Routers
• Access routers are used to connect home or business LANs to
an Internet access link to an Internet Service Provider (ISP)
router.
• Backbone routers transport packets across the Internet
backbone.
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Now that we defined a router
Understanding a Subnet
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What’s a subnet?
• A subnet is a group of devices that can all send packets to each
other without going through a router.