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Bits&Bytes: Week 2: The Network Layer
Terms in this set (32)
At the network layer, i.e. under the IP protocol,...
a packet is usually referred to as an IP datagram.
What is an IP datagram?
A highly structured series of fields that are strictly defined.
Indicates what version of Internet Protocol is being used. 4 bits longs.
Header Length Field
A 4 bit field that specifies how long the the entire header is.
What is the minimum length of a IP header?
Service Type field
These eight bits can be used to specify details about quality of service or QoS technologies.
What is the important takeaway about QoS or quality of service technologies?
That there are services that allow routers to make decisions about which IP datagram may be more important than others.
Total length field
Indicates the total length of the IP datagram it's attached to. Length of field is 2 bytes (or 16 bits).
What is the maximum size of a datagram?
"65,535" which also is the largest number you can represent with 16 bits.
If the total amount of data that needs to be sent is larger than what can fit in a single datagram,...
Then the IP layer needs to split this data up into many individual packets. When this happens, the identification field is used so that the receiving end understands that every packet with the same value in that field is part of the same transmission.
Used to indicate if a datagram is allowed to be fragmented, or to indicate that the datagram has already been fragmented.
What is fragmentation?
The process of taking a single IP datagram and splitting it up into several smaller datagrams.
Fragmentation offset field
Contains values used by the receiving end to take all the parts of a fragmented packet and put them back together in the correct order.
Time to Live or TTL field
An 8-bit field that indicates how many router hops a datagram can traverse before it's thrown away.
What is the main purpose of the TTL field?
To make sure that when there's a misconfiguration in routing that causes an endless loop, datagrams don't spend all eternity trying to reach their destination. An endless loop could be when router A thinks router B is the next hop, and router B thinks router A is the next hop.
Another 8-bit field that contains data about what transport layer protocol is being used, such as TCP or UDP.
Header Checksum Field
Is a checksum of the contents of the entire IP datagram header. Since the TTL field has to be recomputed at every router that a datagram touches, the checksum field necessarily changes, too.
What is a checksum?
A digit representing the sum of the correct digits in a piece of stored or transmitted digital data, against which later comparisons can be made to detect errors in the data.
IP options field
Just a series of zeros used to ensure the header is the correct total size, since after all the IP options field is both optional and variable in length.
An IP datagram is encapsulated in the payload of a frame for transmission.
The entire contents of an IP datagram are encapsulated as the payload of an Ethernet frame. Our IP datagram also has a payload section.
IP addresses can be split into two sections, the network ID, and the host ID.
Earlier, we mentioned that IBM owns all IP addresses that having a nine as the value of the first octet in an IP address. If we take an example IP address of 220.127.116.11, the network ID would be the first octet, and the host ID, would be the second, third and fourth octets.
Address class system
Is a way of defining how the global IP address space is split up.
What the three primary types of address classes?
Class A, class B, and class C.
Class A addresses
Are those where the first octet is used for the network ID, and the last three are used for the host ID. (A class A network has a total of 24 bits of host ID space, this comes out to two to the twenty-fourth, or 16,777,216 individual addresses.)
Class B addresses
Are where the first two octets are used for the network ID, and the second two, are used for the host ID.
Class C addresses
Are those where the first three octets are used for the network ID, and only the final octet is used for the host ID. A class C network, which only has eight bits of host ID space. For a class C network, this comes out to two to the eighth, or 256 addresses.
What are Class D addresses used for?
Are used for multicasting, which is how a single IP datagram can be sent to an entire network at once. These addresses begin with decimal values between 223 and 239.
What has the address class system been replaced by?
By CIDR or Classless inter-domain routing
How do MAC addresses @ the Data Link Layer & IP addresses @ the Network Layer relate to each other?
By the Address Resolution Protocol or ARP
Address Resolution Protocol or ARP
Is a protocol used to discover the hardware address of a node with a certain IP address.
A database of records that maps MAC addresses to IP addresses. The ARP table is stored on a computer's hard disk where it is used by the ARP utility to supply the MAC addresses of network nodes, given their IP addresses.
Why do ARP entries generally expire after a short amount of time?
To ensure changes in the network are accounted for.
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