下面哪个字段包含在TCP头部和UDP头部？ __（16）__。

（75）

（73）

（74）

（72）

（70）

71.The defacto standard Application Program Interface（API）for TCP/IP applications is the“sockets”interface. Although this API was developed for __（71）__ in the early 1980s it has also been implemented on a wide variety of non-Unix systems。TCP/IP __（72）__ written using the sockets API have in the past enjoyed a high degree of portability and we would like the same __（73）__ with IPv6 applications。But changes are required to the sockets API to support IPv6 and this memo describes these changes。These include a new socket address structure to carry IPv6 __（74）__，new address conversion functions，and some new socket options。These extensions are designed to provide access to the basic IPv6 features required by TCP and UDP applications，including multicasting，while introducing a minimum of change into the system and providing complete __（75）__ for existing IPv4 applications。

（69）

PC机不能接入因特网，这时采用抓包工具捕获的以太网接口发出的信息如下： 可以看出该PC机的IP地址为__（68）__，默认网关IP地址为为__（69）__。PC不能接入Internet的原因可能是__（70）__。

（67）

IEEE802.11标准采用的工作频段是__（66）__，下列标准中采用双频工作模式的是__（67）__。