The access layer is the most basic level of the network. It is the part of the network in which people gain access to other hosts and to share files and printers. The access layer is composed of host devices, as well as the first line of networking devices to which they are attached.

Networking devices enable us to connect many hosts with each other and also provide those hosts access to services offered over the network. Unlike the simple network consisting of two hosts connected by a single cable, in the access layer, each host is connected to a networking device. This type of connectivity is shown in the graphic.

Within an Ethernet network, each host is able to connect directly to an access layer networking device using a point-to-point cable. These cables are manufactured to meet specific Ethernet standards. Each cable is plugged into a host NIC and then into a port on the networking device. There are several networking devices that can be used to connect hosts at the access layer, including Ethernet hubs and switches.

A hub is one type of networking device that is installed at the access layer of an Ethernet network. Hubs contain multiple ports that are used to connect hosts to the network. Hubs are simple devices that do not have the necessary electronics to decode the message sent between hosts on the network. Hubs cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports.

Remember that the NIC on a host accepts messages only addressed to the correct MAC address. Hosts ignore messages that are not addressed to them. Only the host specified in the destination address of the message processes the message and responds to the sender.
Only one message can be sent through an Ethernet hub at a time. It is possible for two or more hosts connected to a hub to attempt to send a message at the same time. If this happens, the electronic signals that make up the messages collide with each other at the hub.

A collision causes the messages to become garbled and unreadable by the hosts. A hub does not decode the messages; therefore it does not detect that the message is garbled and repeats it out all the ports. The area of the network where a host can receive a garbled message resulting from a collision is known as a collision domain.

接入层是网络的最基本层， 是用户访问其它主机及共享文件与打印机的网络部分。 接入层包括主机设备及与之连接的第一级网络设备。

网络设备可以使多台主机相互连接， 同时让这些主机访问网络中提供的服务。 与单根电缆连接的两台主机组成的简单网络不同，在接入层中，每台主机都连接至网络设备。 此种连接方式如图所示。

在以太网中，每台主机都可以用点对点的电缆直接与接入层网络设备相连。 这种电缆按以太网标准定制。 每根电缆插入一个网卡后，再插入网络设备的端口。 有多种网络设备用来连接至接入层中的主机，其中包括以太网集线器和交换机。

集线器是一种安装在以太网接入层的网络设备，其具有多个端口，用来连接主机至网络。 集线器是一种简单设备，没有解码网络主机之间传递消息的电子元件。 集线器无法确定哪台主机应接受某个特定的消息。 集线器只是由一个端口接收电子信号，然后在所有其它端口重复生成（或重复）同一消息后发出。

请记住主机上的网卡只接受发送到正确MAC地址的消息，而忽略不是发送给它们的消息。 只有消息目的地址所指定的主机才会处理消息，并回应发送方。

每次只能通过以太网集线器发送一条消息， 所以有可能两台或更多台与一个集线器连接的主机试图同时发送一条消息。 这时组成消息的信号会在集线器相互冲突。

冲突令消息形成乱码，使主机无法理解。 集线器不会对消息解码，因此无法确定该消息是否已乱码，只能在所有端口重复该消息发出。 主机收到因冲突而造成乱码的消息的网络区域被称为冲突域。