5. Intangible Assets

The following table summarizes the gross carrying amount and accumulated amortization for intangible assets resulting from acquisitions (in thousands):

	Existing			Customer
	Technology			Relationship			Total
Amortization period (in years)		11.75		4.0 – 11.75
Gross carrying amount at December 31, 2016	$	4,600		$	10,639		$	15,239
Accumulated amortization		(2,809	)		(6,610	)		(9,419	)
Intangible Assets, net at December 31, 2016	$	1,791		$	4,029		$	5,820
Gross carrying amount at December 31, 2017	$	4,600		$	10,639		$	15,239
Accumulated amortization		(3,257	)		(7,617	)		(10,874	)
Intangible Assets, net at December 31, 2017	$	1,343		$	3,022		$	4,365

Each period, the Company evaluates the estimated remaining useful lives of purchased intangible assets and whether events or changes in circumstances warrant a revision to the remaining period of amortization. If a revision to the remaining period of amortization is warranted, amortization is prospectively adjusted over the remaining useful life of the intangible asset. Intangible assets subject to amortization are amortized on a straight-line basis over their useful lives as outlined in the table above. The Company performs an evaluation of its amortizable intangible assets for impairment at the end of each reporting period. The Company did not identify any impairment indicators during the years ended December 31, 2017 and 2016.

The following table illustrates the amortization expense included in the consolidated statements of operations for the years ended December 31, 2017 and 2016 (in thousands):

	Year Ended December 31,
	2017		2016
Cost of revenue	$	448	$	448
Selling and marketing		1,007		1,007
Total	$	1,455	$	1,455

The estimated annual future amortization expense for purchased intangible assets with definite lives over the next five years is as follows (in thousands):

2018	$	1,455
2019		1,455
2020		1,455
Thereafter		—
Total	$	4,365