Cisco CCNA IP Addressing and Subnetting Part I Cisco CCNA IP Addressing & Subnetting One of the most important topics in any discussion of TCP/IP is IP addressing. An IP address is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network. An IP address is a software address (logical address), not a hardware address. IP addressing was designed to allow a host on one network to communicate with a host on a different network, regardless of the type of LANs the hosts are connected to. Cisco CCNA Introduction to TCP/IP
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Cisco CCNA IP Addressing and Subnetting Part I
Cisco CCNA IP Addressing & Subnetting
One of the most important topics in any discussion of TCP/IP is IP addressing. An IP addressis a numeric identifier assigned to each machine on an IP network. It designates the specificlocation of a device on the network.
An IP address is a software address (logical address), not a hardware address. IP addressingwas designed to allow a host on one network to communicate with a host on a differentnetwork, regardless of the type of LANs the hosts are connected to.
Cisco CCNA Introduction to TCP/IP
The Transmission Control Protocol/Internet Protocol (TCP/IP) suite was created by theDepartment of Defense (DoD) to ensure and preserve data integrity, as well as maintaincommunications in the event of catastrophic war.
So it follows that if designed and implemented correctly, a TCP/IP network can be a trulydependable and resilient one.
The Internet is built on a TCP/IP network.
Cisco CCNA Internet Protocol (IP)
Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merelyexist to support it. IP holds the big picture and could be said to “see all,” in that it’s aware ofall the interconnected networks. It can do this because all the machines on the network have asoftware, or logical, address called an IP address, which we’ll cover more thoroughly later inthis chapter.
IP looks at each packet’s address. Then, using a routing table, decides where a packet is to besent next, choosing the best path.
Cisco CCNA IPv4 Header
The fields in an IPv4 header are as follows:
Version - Indicates the version of IP currently used, currently 4
IP header length - Indicates the datagram header length in 32-bit words
Type of service (TOS) - Specifies how a particular upper layer protocol would like thedatagram to be handled.
Total length - Length of the entire IP packet in bytes including data and header.
Identification - Used to help piece together data fragments. Contains an integer to identifythe current datagram.
Flags - Three bit field used for fragmentation.
Fragment offset -Offset in the original datagram of the data being carried. Measured in 8octets
Time to live (TTL) - Counter that is decremented as a packet traverses the network. Used tokeep packets from looping endlessly.
Protocol - Indicates which upper layer protocol receives incoming packet after IP processingis complete.
Header checksum - Helps ensure IP header integrity.
Source address - IP address of sending node.
Destination address - IP address of receiving node.
Options - Allows support of various options.
Data - Contains upper layer information.
Cisco CCNA User Datagram Protocol vs. Transmission Control Protocol
Reliable (Connection Oriented) – TCP is a reliable protocol that resides at the transportlayer of the OSI reference model. It accounts for retransmission of lost data guaranteeingreliable delivery while also providing sequencing of packets so they can be re-orderedaccounting for packet received out of order. Examples of applications that utilize TCP as atransport are HTTP, E-mail and FTP just to name a few.
Best Effort (Connectionless) – UDP is a best effort protocol that resides at the transport layerof the OSI reference model. It has much less overhead than TCP. It does not retransmitpackets lost in transit nor does it provide sequencing to account for packets received out oforder. A couple of examples of applications that utilize UDP are Voice over IP an VideoStreaming.
Cisco CCNA User Datagram Protocol (UDP)
If you were to compare User Datagram Protocol (UDP) with TCP, the former is basically thescaled-down economy model that’s sometimes referred to as a thin protocol.
UDP doesn’t offer all the bells and whistles of TCP, but it does do a fabulous job oftransporting information that doesn’t require reliable delivery—and it does so using far fewernetwork resources.
Like TCP, UDP resides at layer 4 of the OSI model and utilizes IP as the transport. It is aconnectionless protocol that does not have windowing, sequencing or acknowledgementswhich are the things that make TCP a reliable protocol and UDP not a reliable protocol.
Cisco CCNA UDP Header
The fields in a UDP header are as follows:
UDP Source port - Optional, when specified, identifies the UDP source port. If notspecified, should be zero.
UDP Destination port - Identifies the UDP destination port.
UDP Message Length - The number of octets that comprise user data and the UDP header.
UDP Checksum - Optional, a value of zero means the checksum was not used. Provides away to ensure the data arrived intact.
Data - User data.
Cisco CCNA Transmission Control Protocol (TCP)
Since the upper layers just send a data stream to the protocols in the Transport layers, theInternet layer then routes the segments as packets through an internetwork.
The packets are handed to the receiving host’s Host-to-Host layer protocol, which rebuilds thedata stream to hand to the upper-layer applications or protocols.
TCP creates a reliable sessions by setting up a virtual circuit (TCP connection), whichincludes acknowledgements, sequence numbers and windowing (flow control). TCP utilizes athree-way handshake to establish the TCP connection. The connection is uniquely identifiedby a combination of source ip address/port number and destination ip address/port number.
Cisco CCNA TCP Header
The fields in a TCP header are as follows:
Source port - Identifies the TCP source port.
Destination port - Identifies the TCP destination port.
Sequence number - Usually specifies the number assigned to the first byte of data in thecurrent message. On connection establishment, identifies the initial sequence number to beused in the connection.
Acknowledgement number - Contains sequence number of the next byte of data the senderof the packet expects to receive.
Data offset - Indicates the number of 32 bit words in the TCP header.
Reserved - For future use.
Flags - Control information.
Window - Specifies the size of the sender’s receive window or in other words buffer space.
Checksum - Indicates whether the header was damaged in transit.
Urgent pointer - Points to the first urgent data byte in the packet.
Originating-source port numbers are typically dynamically assigned by the source host andwill equal some number starting at 1024 up through 65535. Port numbers with a value of1023 and below are defined in RFC 1700, which discusses what are called well-known portnumbers.
Virtual circuits that don’t use an application with a well-known port number are assigned portnumbers randomly from a specific range instead. These port numbers identify the source anddestination host in the TCP segment.
The different port numbers that can be used are:
Numbers below 1024 are considered well-known port numbers and are defined in RFC 1700.
Numbers 1024 through 65535 are used by the upper layers to set up sessions with other hosts,and by TCP to use as source and destination addresses in the TCP segment.
Cisco CCNA Setting Up A Reliable Session (Virtual Circuit)
In reliable transport operation, one device first establishes a connection-oriented session withits peer system. This is called a call setup, or a three-way handshake.
Data is then transferred, and when finished, a call termination takes place to tear down thevirtual circuit.
TCP uses a three-way handshake to establish a connection. The TCP three-way handshake isdescribed in detail on the following slide.
Cisco CCNA TCP Connection Establishment
Shown in the slide is the TCP three way handshake used in establishing all TCP connections.The important thing here is the bits that are set with each packet (i.e. SYN, SYN+ACK,ACK).
As depicted, to establish a connection, TCP uses a three-way handshake. Before a clientattempts to connect with a server, the server must first bind to a port to open it up forconnections: this is called a passive open. Once the passive open is established, a client mayinitiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:
1. The active open is performed by the client sending a SYN to the server. It sets thesegment's sequence number to a random value A.
2. In response, the server replies with a SYN-ACK. The acknowledgment number is set to onemore than the received sequence number (A + 1), and the sequence number that the serverchooses for the packet is another random number, B.
3. Finally, the client sends an ACK back to the server. The sequence number is set to thereceived acknowledgement value i.e. A + 1, and the acknowledgement number is set to onemore than the received sequence number i.e. B + 1.
At this point, both the client and server have received an acknowledgment of the connection.
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