welcome to the Cisco netacad introduction to networks video series by
Jason Johnson this video covers module eight part one
of two and it's going to be covering the network layer in module eight part one
of two we will be looking at Network layer characteristics and we'll explain
how the network layer uses IP protocols for reliable communications we will also
look at the v4 packet which explains the role of the major header fields in the
ipv4 packet and then the ipv6 packet the same things we're going to be looking at
the fields in the v6 packet Network layer characteristics so the network
layer is layer three so we've talked about the physical layer the datalink
layer and now we're going to be looking at the network layer so the network
layer is going to be providing routing it provides services to allow n devices
to exchange data the IP V version 4 or v4 and IP V version 6 or I'll call it as
v6 are the principal network layer communication protocols at this layer
the network layer performs four basic operations addressing end devices
encapsulation routing and D encapsulation and we can look over here
at the image here so we see that on OSI model there it's on the third layer and
the layer 3 will do the encapsulation on its segment it sends it on down to the
medium sends it across and then comes back up and D encapsulates it there at
the layer 3 on that end IP encapsulates the transport layer segment IP can
either use v4 or v6 and not impact the layer 4 segment so the IP packet will be
examined by all layer 3 devices as it moves along the network the IP
addressing does not change source from source to destination so the transport
layer encapsulation you have the segment header the data it goes down and the
layer 3 will then encapsulate that layer a few of the characteristics of IP or
Internet Protocol it's meant to have low overhead and may be described as
connectionless best-effort and media independent what we mean by IP
is connectionless is that IP does not establish a connection with the
destination before sending the packet it just sends them there's no control
information needed there's no synchronization no
acknowledgments no back-and-forth the package just gets sent the destination
will receive the packet when it arrives but no pre notifications are sent by the
IP if there is a need for connection oriented traffic then another protocol
will handle this typically TCP at the transport layer IP
is best-effort IP will not guarantee delivery of the packet it just don't
care it just sends it you don't care if it gets there not IP has reduced
overhead since there's no mechanism to resend data that's not received IP does
not expect acknowledgments back and IP does not know if the other device is
operational or if its receive the packet again it just sends it forgets it and it
don't care so the IP packets get sent they get routed best effort and they get
there or they don't the other end sees them or they don't and some packets can
be lost in route and it doesn't matter IP is unreliable of course we just
discussed that IP has best effort and connectionless so it's unreliable that
it cannot manage or fix undelivered or corrupted packets it cannot transmit
after retransmit after an error IP cannot realign out of sequence packets
and IP must rely on other protocols for those functions now IP is media
independent it does not concern itself with the type of frame required at the
data link layer or the media type at the physical layer IP can be sent over any
media type copper fiber Wireless any of them now the network layer will
establish the maximum transmission unit or the MTU you'll see it you'll hear it
shortened to MTU the network layer receives this from the control
information sent by the data link layer the network then establishes the MTU
size fragmentation is when layer 3 splits the v4 packet into smaller units
so fragmenting causes latency v6 does fragment packets an example as a router
goes from Ethernet to a slow man with a smaller MTU and so it will fragment a v4
but it will not do that with v6 let's take a look at the v4 packet v4 is the
primary communication protocol for the network layer the network header has
many purposes it ensures the packet is sent in the correct direction to the
destination it contains information for network layer processing in various
fields and the information in the header is used by all layer 3 devices that
handle the packet the v4 network header characteristics it is in binary it
contains several fields of information the diagram that we're looking at here
is read from left to right 4 bytes per line and the two most important fields
with a source and destination so you have your version your total length your
flags your freight fragment offset your time-to-live your protocols your headers
checksum and your source and IP address or your source and destination IP
address and protocols may have one or more functions in there now significant
fields in the v4 header diversion there will be a v4 as opposed to a v6 for ipv6
a 4 bit field is 0 1 0 0 differentiated services so that's used for quality of
service or diffserv DS field or other in serv TOS or type of service the texts
were corruption in the v4 header that's the header checksum the time-to-live the
layer 3 hop count that's how many hops it's made when it becomes 0 the router
will discard the packet that is put in place to keep make sure that packets
don't just bounce around the internet forever so there's a time to live on a
packet and it will go and once it gets to its hop limit or it's time to live it
will just die it'll get discarded at the next router the protocol IDs next level
protocol ICMP TCP UDP it's a 32-bit source address and a 32-bit destination
address these six packets v4 has some major limitations to it it's a draw
are depleted we ran out of them a long time ago so we have to do something
different so there's a lack of end-to-end connectivity with v4 so to
make sure v4 survived this long private addressing and network address
translation or NAT was created that ended direct communications with public
addressing because you are translating IP addressing at the router with network
address translation translation increased network complexity NAT was
meant as a temporary solution and it creates issues on a network as a side
effect of manipulating the network headers addressing and NAT causes
latency and troubleshooting issues so let's look at a v6 overview here v6 was
developed by the internet Engineering Task Force RIT ETF it overcomes the
limitations of v4 it provides increased address space it's based on 128 bit
address not 32 bits it improved or simplified the header with fewer fields
and it eliminated the need for NAT since there's a huge amount of addressing I
mean we'll take a look here in just a second there's a lot there's no need for
private addressing internally and it can be mapped to a shared public address and
let's take a look at what size we're looking at here just to wrap your head
around the size of v6 addressing 1 billion is 10 to the ninth power and
that's what 10 1 billion looks like it is 1 undecillion that's how many v6
addresses you can have or that are available 10 to the 36 power it's a big
number I've heard it compared a lot of
different ways grains of sand on the planet I mean there it's it's just a
very large number the ipv4 packet header fields in the v6 packet header the v6
header is simplified but not smaller the header is fixed at 40 bytes or octet
long several v4 fields were a move to improve performance and the four fields
were removed to improve before and those that were in removed are the flag the
fragment offset and the header checksum significant feels in the v6 header the
version there will be a 6 as opposed to a 4
the traffic class is used for QoS the flow label informs device informs
devices to handle identical flow labels the same way 20-bit filled the payload
length the 16-bit field indicates the length of the data portion or payload of
the v6 packet the next header IDs next level protocol ICMP TCP UDP the hop
limit that replaces the time-to-live field and the source v6 address that's
128-bit source address and the destination v6 address it's 128-bit
destination address the v6 packet may contain extension headers you can
provide optional Network layer information they're placed between the
v6 header and the payload and they may be used for fragmentation security
mobility and support unlike v4 routers do not fragment v6 packets though all
right this has been module eight part 1 of 2 the next video module 8 part 2 of 2
you should be able to find the link below in the description and I hope this
information was helpful for you and have a great day